Method for making multilayer polyester film

ABSTRACT

A multilayer polyester film, and a method for making the same, is provided. The film consists of alternating layers of polyethylene terephthalate and polyethylene naphthalate. Biaxial orientation and subsequent restrained heat setting of these materials results in thin films with tensile moduli in both stretch directions well in excess of the values obtained with monolithic films of either material. In some embodiments, a slippery surface is imparted to the film without the use of conventional slip agents.

FIELD OF THE INVENTION

The present invention relates to multilayer films, and in particular tomultilayer films comprising a plurality of layers of naphthalenedicarboxylic acid polyester and terephthalic acid polyester.

BACKGROUND OF THE INVENTION

Polyester films of various compositions are known to the art. Thesefilms, which may be continuously extruded into sheets of variousthicknesses, have good tensile strength and modulus, and have found use,among other things, as magnetic media substrates.

To date, much attention in the art has been focused on the opticalproperties of multilayer films. Alfrey et al., Polymer Engineering andScience, Vol. 9, No. 6, pp. 400-404 (November 1969), Radford et al.,Polymer Engineering and Science, Vol. 13, No. 3, pp. 216-221 (May 1973),and U.S. Pat. No. 3,610,729 (Rogers), for example, describe thereflectivity of certain multilayer polymeric films. This work has beenextended to multilayer polyester films. Thus, U.S. Pat. No. 3,801,429(Schrenk et al.) and U.S. Pat. No. 3,565,985 (Schrenk et al.) disclosemultilayer composites made from various resins, including polyesters,and methods for making the same. The composites have the property ofbeing iridescent, even without the addition of pigments.

U.S. Pat. No. 4,310,584 (Cooper et al.) describe the use of polyestersin making multilayer iridescent light-reflecting film. The film includesalternating layers of a high refractive index polymer and a polymer witha low refractive index. The high refractive index polymer is a castnonoriented film that includes a thermoplastic polyester or copolyestersuch as polyethylene terephthalate (PET), polybutylene terephthalate andvarious thermoplastic copolyesters which are synthesized using more thanone glycol and/or more than one dibasic acid.

U.S. Pat. No. 5,122,905 (Wheatley) describes a multilayer reflectivefilm with first and second diverse polymeric materials in alternatinglayers that exhibits at least 30% reflection of incident light. Theindividual layers have an optical thickness of at least 0.45micrometers, and adjacent layers have a refractive index difference ofat least 0.03. U.S. Pat. No. 5,122,906 (Wheatley) describes similarreflecting bodies, wherein a substantial majority of individual layershave an optical thickness of not more than 0.09 micrometers or not lessthan 0.45 micrometers, and adjacent layers have a refractive index of atleast 0.03.

Some attempts have also been made to improve the mechanical propertiesof particular multilayer films. Thus, U.S. Pat. No. 5,077,121 (Harrisonet al.) describes polyethylene-based multilayer films consisting oflayers of two or more different resins, wherein the draw ratios of thecomposite film are found to exceed the draw ratios of monolithic filmsof the component materials. In the films described, a layer of highelongation, low modulus material is sandwiched between layers of lowelongation, low modulus material. The reference also notes that asimilar phenomenon is sometimes observed in composites wherein a highmodulus, low elongation material is sandwiched between layers of highelongation material, although in many of these composites, the lowelongation material fails at its characteristic low elongation, causinga simultaneous, premature failure of the high elongation layers.

To date, however, relatively few improvements have been made in themechanical properties of multilayer polyester films, despite the factthat such films have become increasingly important in a wide variety ofcommercial applications. While polyester films are already availablewhich have a high modulus and medium elongation, in a variety of uses,as when polyester films are used as engineering materials or are subjectto winding operations, the physical limitations of these films arealready being tested. There thus remains a need in the art for amultilayer polyester film having improved mechanical properties, and fora method of making the same. In particular, there is a need in the artfor multilayer polyester films having improved tensile modulus, tensilestrength, and stretchability.

A further problem encountered with polyester films, and frequentlycommented on in the literature, relates to the incidence of hazing.Hazing in polyester films is undesirable in applications where a clearfilm would be preferred, as in window films. In other applications, aparticular degree of hazing is acceptable or even desirable. To date,however, the phenomenon of hazing has been poorly understood, and nomethods have been provided which allow for easy control of the degree ofhazing in polyester films. There is thus a need in the art for a methodof controlling the degree of hazing in polyester films, and particularlyin multilayer polyester films. In particular, there is a need in the artfor a method of producing multilayer polyester films with any desireddegree of hazing, through manipulation of readily controllable processparameters.

Yet another problem encountered in polyester films relates to theircoefficient of friction. Thin polyester films having a high coefficientof friction are prone to wrinkling, web breaks, and similar damageduring winding and handling. In these applications, it would bedesirable to use a polyester film having a lower coefficient offriction, so that adjacent surfaces of the film would slide over eachother easily.

To date, this has been accomplished through the use of slip agents.However, the use of slip agents is undesirable in that it complicatesthe manufacturing process, and frequently compromises the mechanical oroptical properties of the resulting film. There is thus a need in theart for polyester films which are substantially devoid of slip agents,but which have a comparatively low coefficient of friction. There isalso a need in the art for a method of controlling the coefficient offriction in a polyester film without the addition of slip agents.

These and other needs are met by the present invention, as hereinafterdisclosed.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a new class of polyestermultilayer films, and to a method for making the same. Surprisingly, ithas been found that, by extruding a film having alternating layers ofpolyethylene naphthalate (PEN) and polyethylene terepthalate (PET), amultilayer composite is obtained which can be stretched to a higher drawratio than monolithic films of comparable dimensions of either PEN orPET. Upon orientation, the multilayer film has a tensile modulus andtensile strength superior to that of monolithic films of PEN or PET. Thecomposite structure permits the PET layers within the film to remainstretchable even after they have crystallized. Remarkably, the optimumstretching temperature for these films is found to be significantlyhigher than the glass transition temperature of either component resin.By contrast, the optimum stretching temperature for monolithic films ofeach component resin are known in the art to be only slightly above Tg.

In another aspect, the present invention relates to a method by whichmultilayer polyester films having a desired degree of hazing may beproduced in a continuous or noncontinuous manner, at variouscombinations of intrinsic viscosities and at various ratios of PEN toPET, and with either PET or PEN as the surface resin. Surprisingly, ithas been found that the degree of haze in the finished stretched filmcan be controlled through proper manipulation of preheating temperatureand duration. Thus, the method allows films to be produced with anydesired degree of clarity. Various other features of the films,including shrinkage, friction, color, and modulus, may also becontrolled through manipulation of these and other parameters.

In yet another aspect, the present invention relates to polyester filmshaving a desired degree of surface roughness, and to a method for makingthe same. Surprisingly, it has been found that the degree ofcrystallization of PET in a multilayer film comprising layers of PET andPEN can be used to manipulate the degree of surface roughness so as toprovide a polyester film that has a slippery surface without theaddition of slip agents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic drawing of a first embodiment of the multilayerfilm of the present invention;

FIG. 1b is a schematic drawing of a second embodiment of the multilayerfilm of the present invention;

FIG. 2 is a graph comparing the modulus as a function of biaxial drawratio of a pure PEN film to that of a 29 layer film consisting of 80% byweight PET and 20% by weight PEN;

FIG. 3 is a graph of the ultimate biaxial draw ratio of the films of thepresent invention as a function of multilayer composition;

FIG. 4 is a graph of the effect of heat setting on the films of thepresent invention;

FIG. 5 is a graph of the modulus as a function of PEN fraction for 29layer films of the present invention;

FIG. 6 is a graph of the modulus as a function of PEN fraction for 29layer films of the present invention;

FIG. 7 is a graph of the maximum draw ratio as a function of drawtemperature for various 29 layer films of differing PEN:PET ratios;

FIG. 8 is a graph of the modulus (at the maximum draw ratio) as afunction of draw temperature for two 29 layer films of differing PEN:PETratios;

FIG. 9a is a three dimensional interferometry plot of side 1 of Example135;

FIG. 9b is a three dimensional interferometry plot of side 2 of Example135;

FIG. 10a is a three dimensional interferometry plot of side 1 of Example136;

FIG. 10b is a three dimensional interferometry plot of side 2 of Example136;

FIG. 11a is a three dimensional interferometry plot of side 1 of Example137;

FIG. 11b is a three dimensional interferometry plot of side 2 of Example137;

FIG. 12a is a three dimensional interferometry plot of side 1 of Example138;

FIG. 12b is a three dimensional interferometry plot of side 2 of Example138;

FIG. 13a is a three dimensional interferometry plot of side 1 of Example139;

FIG. 13b is a three dimensional interferometry plot of side 1 of Example139;

FIG. 14a is a three dimensional interferometry plot of side 1 of Example141;

FIG. 14b is a three dimensional interferometry plot of side 1 of Example141;

FIG. 15 is a graph depicting the engineering stress as a function ofdraw ratio for Examples 202 and 203; and

FIG. 16 is a graph depicting the engineering stress as a function ofdraw ratio for Examples 202 and 203.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a conventional "tenter" film process, one or more polymers areextruded onto a temperature-controlled roll (or "casting wheel") in theform of a continuous film or sheet. This film or sheet, prior to anyorientational stretching in either the machine direction or transverse(cross) direction, is often referred to by the term "cast web". As usedherein, the terms "film" and "web" are used interchangeably to refer tothe polymer sheet at any point in the process subsequent to casting onthe casting wheel, but the term "cast web" is reserved for film whichhas not yet experienced significant orientational stretching in eitherthe machine or transverse direction.

As indicated in FIGS. 1a-b, the multilayer films 10 of the presentinvention are formed from at least two different polymer resins. Theseresins are coextruded into a composite film having alternating layers ofa first resin 12 and a second resin 14. Preferably, either the first andsecond resins are immiscible, or the coextrudate is rapidly cooled to atemperature below the glass transition temperatures of the resins soonafter the first and second resins come into contact with one anotherinside the coextrusion equipment. The satisfaction of one of these twocriteria ensures that adjacent layers in the composite film are joinedacross an interface 16, which may be either sharp or diffuse.

The films of the present invention may contain virtually any number oflayers greater than or equal to three. However, there are preferably atleast 7 layers in the finished film, and more preferably at least 13layers. The presence of at least 7 or 13 layers in the film is found tocoincide with the onset of certain desirable properties, such asimprovements in orientational stretchability, modulus, and surfaceroughness. Typically, the films of the invention will contain only a fewdozen layers, although finished films containing hundreds, or eventhousands, of layers are found to be advantageous in some applications.

The layers of different resins are preferably arranged in an alternatingsequence in at least a portion of the film, and preferably throughoutthe film as a whole. However, in some embodiments, as in the embodimentdepicted in FIG. 1b, the film may be extruded with one or more adjacentlayers of the same resin. In most conventional extrusion processes,adjacent layers of the same resin will coalesce into a single layer ofgreater thickness. This tendency may be used to produce doubly thicklayers where the provision of such layers is desirable, as on thesurfaces of some films.

The relationships among the thicknesses of the various layers is notlimited. Layers of the first resin may be different in thickness thanlayers of the second resin. Different layers of the same resin may alsobe of different thicknesses.

The present invention also allows for virtually any number of layers ofany number of different resins to be incorporated into the multilayerfilm. Thus, while the multilayer films of the present invention willmost commonly contain only two types of layers made from two differentresins, the invention also contemplates embodiments wherein three ormore different resin types are present in the finished film.

Many different polymer resins can be used to make multilayer films inaccordance with the present invention. However, as noted above, it ispreferred that resins and/or processing conditions be chosen so as tomaintain the separate chemical identity of the layers across aninterface between each pair of adjacent layers.

The present invention contemplates that any polymer resinsmelt-processable into film form may be used. These may include, but arenot limited to, homopolymers and copolymers from the following families:polyesters, such as polyethylene terephthalate (PET), polybutyleneterephthalate, poly (1,4-cyclohexylenedimethylene terephthalate),polyethylene bibenzoate, and polyethylene naphthalate (PEN); liquidcrystalline polyesters; polyarylates; polyamides, such as polyamide 6,polyamide 11, polyamide 12, polyamide 46, polyamide 66, polyamide 69,polyamide 610, and polyamide 612; aromatic polyamides andpolyphthalamides; thermoplastic polyimides; polyetherimides;polycarbonates, such as the polycarbonate of bisphenol A; polyolefins,such as polyethylene, polypropylene, and poly-4-methyl-1-pentene;ionomers such as Surlyn™ (available from E.I. du Pont de Nemours & Co.,Wilmington, Del.); polyvinyl alcohol and ethylene-vinyl alcoholcopolymers; acrylic and methacrylic polymers such as polymethylmethacrylate; fluoropolymers, such as polyvinylidene fluoride, polyvinylfluoride, polychlorotrifluoroethylene, and poly(ethylene-alt-chlorotrifluoroethylene); chlorinated polymers, such aspolyvinyl chloride and polyvinylidene chloride; polyketones, such aspoly(aryl ether ether ketone) (PEEK) and the alternating copolymers ofethylene or propylene with carbon monoxide; polystyrenes of anytacticity, and ring- or chain-substituted polystyrenes; polyethers, suchas polyphenylene oxide, poly(dimethylphenylene oxide), polyethyleneoxide and polyoxymethylene; cellulosics, such as the cellulose acetates;and sulfur-containing polymers such as polyphenylene sulfide,polysulfones, and polyethersulfones.

Films in which at least one of the first resin and the second resin is asemicrystalline thermoplastic, are preferred. More preferred are filmsin which at least one resin is a semicrystalline polyester. Still morepreferred are films in which at least one resin is polyethyleneterephthalate or polyethylene naphthalate. Films comprising polyethyleneterephthalate and polyethylene naphthalate as the first and secondresins are especially preferred, and the films thereof are found to havemany desirable properties, including good orientational stretchability,high modulus, and controllable degrees of surface roughness, even in theabsence of added slip agents. However, the exact choice of resinsultimately depends on the use to which the multilayer films are to beapplied. Thus, for example, if the multilayer film is to be used foroptical applications, other factors, such as the indices of refractionof the resins, must be taken into account. Other pairs of polymer resinswhich provide the orientational stretchability, high modulus, and/orsurface roughness advantages described herein are contemplated by thepresent invention.

Among the polyesters and copolyesters considered suitable for use in thepresent invention are those formed as the reaction product of diols withdicarboxylic acids and/or their esters. Useful diols include ethyleneglycol, propane diol, butane diol, neopentyl glycol, polyethyleneglycol, tetramethylene glycol, diethylene glycol, cyclohexanedimethanol,4-hydroxy diphenol, bisphenol A, 1,8-dihydroxy biphenyl,1,3-bis(2-hydroxyethoxy)benzene, and other aliphatic, aromatic,cycloalkyl and cycloalkenyl diols. Useful dicarboxylic acids includeterephthalic acid, isophthalic acid, any of the isomeric naphthalenedicarboxylic acids, dibenzoic acid, 4,4'-bibenzoic acid, azelaic acid,adipic acid, sebacic acid, or other aliphatic, aromatic, cycloalkane orcycloalkene dicarboxylic acids. Esters of the dicarboxylic acids may beused in place of or in combination with the dicarboxylic acidsthemselves. When polyethylene terephthalate and polyethylene naphthalateare to be used as the first and second resins, either or both maycontain minor amounts of comonomers and/or additives.

The intrinsic viscosity of the polymer resins to be used in the presentinvention is not specifically limited. Depending on the equipment usedfor the extrusion and casting of the multilayer film, the meltviscosities of the polymer resins may need to be matched to greater orlesser degrees of precision. Monolayer films of PET are typically madefrom resins having intrinsic viscosities of about 0.60. These and evenlower IVs may also be accommodated in the present invention. PET resinswith IVs as high as 1.10 or higher may be routinely obtained fromcommercial sources, and may also be used. The PEN resin should be chosenso as to match the selected PET resin in melt viscosity closely enough,so that smooth, defect-free films may be cast with the equipment to beused.

Another aspect of the present invention concerns films having tailorablesurface roughness, haze, and coefficient of friction, without the use ofconventional "slip agents". Tailorable surface roughness is desirable soas to provide films appropriate to diverse applications. For instance,films employed as substrates for magnetic recording media must berelatively smooth on the side or sides to which the magnetic coating isapplied. Typical requirements are for root mean square average surfaceroughness (Rq) of less than 60 nm, with many applications requiring Rqless than 20 nm, and some requiring Rq less than 10 nm. On the otherhand, capacitor films and printable or writeable films must have a highsurface roughness to allow oil impregnation and to accept ink,respectively. Typical requirements in these applications are for Rqvalues greater than 100 nm, with some applications requiring Rq valuesof 200 nm or more.

Haze is well-known in the film industry to correlate with roughness,especially in the absence of complicating factors such as particulateadditives. Furthermore, haze is considerably easier to measure and/orqualitatively assess than is surface roughness. Thus, while of interestin its own right for certain applications, haze was typically assessed,in the experiments described herein, as a means of making qualitativecomparisons of the surface roughnesses of films.

A low coefficient of friction is desirable so as to improve handling andwinding properties of the film during manufacture and use, and toprevent blocking during storage. Thinner films are known to requirelower coefficients of friction in order to be wound and handled withoutdamage such as wrinkling and web breaks. Coefficient of friction alsocorrelates well with surface roughness, provided that composition andconstruction within a series of films remains unchanged. Thus, forpolyethylene terephthalate films containing a given slip agent,increasing the amount of the slip agent increases the surface roughness,and lowers the coefficient of friction in a well-correlated manner. Theform of the correlation may differ for a different slip agent, however.

Slip agents are so named because the purpose of their use in films is toprovide a low coefficient of friction (i.e., slipperiness) required forhandling. Slip agents are defined as inert solid fine particles within,or on, the surface(s) of the film. They may be incorporated into thefilm during its formation, or coated onto the film's surface afterward.When coated on, they may be incorporated in a binder polymer, which mayor may not be the same polymer as the film itself, or they may bedeposited from a dispersing medium or solvent. When incorporated intothe film during its formation, they may be present throughout the film,or only in layers coextruded or laminated on one or both surfaces. Slipagents may be incorporated by blending them into the film polymer resinduring extrusion, or they may be incorporated into the resin during itsmanufacture.

Slip agents may be spherical or non-uniform in shape. They may or maynot form agglomerates. Individual slip agent particles usually aresmaller than 5 microns in diameter, and are most commonly an order ofmagnitude or more smaller than that. They are incorporated into films atup to about 3% by weight, but more typically are present at well under1%.

Slip agents can be polymeric or non-polymeric. Typical examples ofnon-polymeric slip agents are kaolin, talc, silicas, aluminas, metalcarbonates such as calcium carbonate, metal oxides such as titaniumdioxide, silicate salts, metal phosphates, metal sulfates, metaltitanates, metal chromates, metal benzoates, metal terephthalates, formsof carbon such as carbon black, and glasses. Polymeric slip agents maybe crosslinked or non-crosslinked. Typical examples of crosslinkedpolymeric slip agents are silicones, styrenics, acrylics, andpolyesters. Non-crosslinked polymeric slip agents are typicallythermoplastics, and they are processed so as to be finely dispersed asparticles within the film resin. Typical examples of non-crosslinkedpolymeric slip agents are polyolefins, ionomers, styrenics,polycarbonates, acrylics, fluoropolymers, polyamides, polyesters,polyphenylene sulfide, and liquid crystalline polymers.

All conventional slip agents have in common a fine particulate naturein, or on the surface(s) of, the finished film. Furthermore, allconventional slip agents of the type that are incorporated into the filmduring its formation (rather than coated on afterward) have in common afine particulate nature in, or on, the surface(s) of the extruded castweb as well. For this reason, there are significant disadvantages to theuse of slip agents. The use of slip agents necessitates the use offiltration devices in the manufacture of the film. These devices arefrequently clogged by the slip agent. Also, slip agents may formundesirably large agglomerates in the film, which have a negative effectin many applications. Incorporation of inorganic particulates usuallyrequires that they be milled to the appropriate size and/or"classified". These are added steps that are difficult to control andadd cost. Incorporation of crosslinked polymer particles requires eithersimilar preparation, or precise control of particle shape and sizeduring their formation. Incorporation of non-crosslinked polymerparticles requires difficultly-obtained control over their sizedistribution and/or dispersion during film extrusion. Furthermore, theuse of slip agents presents the possibility for the formation of dustand debris, and scratching of the film surface, during biaxialorientation, handling, winding, slitting, converting, processing and/oruse of the film.

For all these reasons, it is desired to control surface roughness andcoefficient of friction in polymer films without resort to the additionof conventional inert solid fine particulate slip agents. Surprisingly,it has been discovered that the multilayer films of the presentinvention possess varying degrees of surface roughness and "slip"(coefficient of friction), even in the absence of slip agents, and thatthe degree of surface roughness and value of coefficient of friction isadjustable by varying process conditions, such as the temperature andduration of preheating prior to orientation.

In the Examples set forth below, the following procedures were used todetermine the physical properties of the films tested.

Intrinsic Viscosity

Intrinsic viscosity was determined identically for both PEN and PET. Thesolvent used is a 60/40 mixture (by weight) of phenol andortho-dichlorobenzene. A temperature of 110° C. is used to effect thedissolution of the polymer in 30 minutes. A size 150 Cannon-Fenskeviscometer is used, and data is taken at 30° C. A single-pointdetermination of relative viscosity is done, using a solutionconcentration of about 0.5% polymer by weight. Relative viscosity is theratio of efflux times in the viscometer for the solution and the puresolvent. The relative viscosity is converted to an approximate value ofintrinsic viscosity using the well-known Billmeyer relationship:

    IV={η(rel)-1+3ln η(rel)!}/4c

where η(rel) is the relative viscosity and c is the polymer solutionconcentration in g/dL.

Modulus Measurement

Modulus was measured on a computerized Instron tensile tester. Specimenswere cut to 0.5 inch width. The gauge length between Instron grips was 4inches. The test was performed at a rate of 2 inch/min crosshead speed.The specimens were cut to approximately 7 inch lengths to permit easymounting in the 1 inch wide Instron grips and great care was taken toavoid either excessive slack or pre-tension for these thin filmspecimens. The thickness for each specimen was determined by taking tenmeasurements within the gauge length. The average of all ten was used incalculations. For films prepared on a continuous film line, specimenswere cut from the center of the web. For films prepared on a laboratoryfilm stretcher, tensile specimens were cut from the center of the squarespecimen from the stretcher. In this case, specimens for determining thetensile properties in the machine direction were taken from one squarestretcher specimen, and specimens for determining the tensile propertiesin the transverse direction were taken from a separate square stretcherspecimen, so that all could be cut from the center. In some evaluations,five specimens were cut and tested, and the values obtained wereaveraged. Variation was small, however, so for most evaluations onlythree specimens were tested and averaged.

In some examples, a value is given for the "Green modulus". It wasdiscovered that the modulus of the films made in these studies increasedover time. While this is not uncommon for biaxially oriented polyesterfilms, in some cases the increase was more dramatic than that which isnormally observed for PET films. Thus, modulus measurements were madeeither as soon as possible (and no more than four hours after the filmwas made), or after at least one week had elapsed. It is believed thatmost if not all of the modulus enhancement or "aging" occurs in theinterim. Measurements taken on "aged" film are referred to simply as"modulus", while measurements taken quickly are referred to as "green"modulus. Most reported values for green modulus represent the average oftwo tests.

Reversible Coefficient of Thermal Expansion

The Reversible Coefficient of Thermal Expansion, or CTE, was measuredusing a Zygo model 121 testing apparatus. A 0.5 inch wide, 12 inch longtest specimen is mounted flat. The temperature differential used fortesting was approximately 20°-25° C., going from Room Temperature toabout 45° C. The CTE is measured as mm of expansion per mm of initiallength per ° C. of temperature change. Since the expansion is typicallyon the order of 1-20×10⁻⁶ in these units, it is reported as parts permillion per ° C. (ppm/° C.). For most films tested, three specimens wereprepared and the results were averaged.

Reversible Coefficient of Hygroscopic Expansion

The Reversible Coefficient of Hygroscopic Expansion, or CHE, wasmeasured on a Neenah Paper Expansimeter. A 0.5 inch (1.27 cm) by 9.5inch (24.13 cm) sample is arranged in the apparatus between a hook and alevel/hook arrangement. A micrometer is used to adjust the level after achange to the test specimen length occurs due to controlled change inthe humidity of the air in the test apparatus. The humidity test rangewas 23-94% relative humidity (% R.H.). CHE is measured as mm ofexpansion per mm of initial length per % R.H. Similarly to the CTE, thevalues for CHE are conveniently expressed as ppm/% R.H. Again, mostresults represent the average of three tests.

Irreversible Thermal Shrinkage

Thermal shrinkage was measured as follows: Test specimens were cut to0.5 inch (1.27 cm) width and 12 inches (30.48 cm) in length. Ink"X"-marks were placed about 10 inches (25.4 cm) apart on each specimen.The exact distance between the two marks was determined by using an"optical comparitor" or "electronic ruler", a device which preciselydetermines the distance traveled by a microscopic eyepiece moved fromone mark to the other. The specimens were then allowed to hangunrestrained in a temperature-controlled oven for 3 days (72 hrs) at 80°C. The specimens were removed from the oven and remeasured. Great careis taken during both measurements to ensure that the specimens aremounted on the optical comparitor flat and straight, and with as littletension as possible. Shrinkage results are expressed as a percent of theoriginal specimen length, and are regarded as accurate to ±0.01%. Heretoo, results are expressed as the average of three tests. In someevaluations, the oven conditions were changed to 3 days residence timeat 65° C. Some measurements were also done for 15 minutes residence timeat 150° C.

Haze

Haze was measured with a Gardner Hazemeter. Model AUX-10 or AUX-10A wasused, with a sample size of approximately 1 inch (2.54 cm) square. Carewas taken to ensure that the film specimens were free from dust,scratches, etc. Light passing through the sample either directly, or"diffused", is captured and quantified by the instrument. Haze is theamount of diffused transmitted light as a percentage of all transmittedlight (direct and diffuse).

Coefficient of Friction

Static and Kinetic Coefficients of Friction were measured with anInstron tensile tester. In this document, all coefficients of frictionare measured on films made to slide with one of their surfaces incontact with the opposite surface. A 2 inch (5.08 cm) wide and 10 inch(25.4 cm) long specimen is cut from the film and mounted on a horizontalplatform. A 1 inch (2.54 cm) wide by 5 inch (12.7 cm) long specimen iscut from the film and mounted on a special 200 gram "sled" with a 0.97inch (2.46 cm) radius. The specimens are cut so that the film's machinedirection is in the long dimension of each specimen. The sled is placedon the platform, and pulled with a chain via a pulley by the Instroncrosshead at 1/2-inch per minute (2.1×10⁻² cm/s). At least 4 inches(10.16 cm) of crosshead travel is used.

The coefficient of friction is defined as the ratio of the FrictionalForce to the sled weight. The Frictional Force is read directly from theInstron recorder chart. The Static Coefficient of Friction is determinedby using the peak force recorded at the beginning of the test. TheKinetic Coefficient of Friction is determined by using the average forcerecorded at long times in the test.

Surface Roughness by Interferometer

Surface roughness is measured on a specially-constructed instrumentutilizing the principles of laser light interferometry. Specimens arecut from the film 1/2-inch (1.27 cm) wide by 6 inches (15.24 cm) long,and are vapor coated with metal. As configured, the system probes anarea about 230 microns wide by 365 microns long. A 3-dimensional imageof the probed area is generated. Statistical parameters of the surfaceare also calculated by the instrument's dedicated computer. Normally,two averages, "Ra" and "Rq", both well known to those experienced insurface profilometry, are reported. Ra is the arithmetic mean height ofdeviations from the hypothetical average plane of the film surface. Rqis the geometric mean height of deviations from the same plane.

Surface Roughness by Rodenstock

In some cases, films of the current invention proved so rough as to beoutside the useful range of the Interferometer, above. Thus, a secondmethod was employed, using the Rodenstock RM600 surface analyzer, acommercially available instrument. The Rodenstock is a non-contactsurface "stylus" which probes the specimen along a 5 mm long line,rather than canvassing a rectangular area, and works on the principle ofdynamically refocusing a laser beam on the traveling film surface.Specimens for Rodenstock must also be vapor coated. The Rodenstocktechnique also calculates Ra and Rq, but due to the way the data iscollected, filtered, and analyzed, it returns consistently higher valuesthan the Interferometer, for the same specimen. Thus, values of Ra andRq from the two instruments cannot be usefully compared.

EXAMPLES 1-24

The following examples demonstrate the ability to coextrude PEN and PETinto multilayer webs at various combinations of intrinsic viscositieswith either polymer at the two film surfaces, throughout the full rangeof relative composition.

Several webs of PEN and PET were cast by coextrusion. The webs consistedof alternating layers (usually 29 total) of PEN and PET, which wereobtained from the Goodyear Chemical Co., Akron, Ohio. In each web, thetwo surface layers (the 1st and 29th) consisted of the same polymer. Asshown in Table 1, in some coextrusions, both of the surface layersconsisted of PEN, while in others, both surface layers consisted of PET.

Several different molecular weights for each resin were used in theexperiments, as reflected in the values for Intrinsic Viscosity reportedin Table 1. The polymers were extruded on separate 13/4" (4.4 cm) singlescrew extruders. PEN was extruded at about 293° C., and PET was extrudedat about 282° C. The throughput of each extruder was adjusted within therange of 5.22 kg/hr (1.45×10⁻³) to about 43.5 kg/hr (1.2×10⁻²) so as toarrive at the polymer proportions shown in Table 1. A film die whichaccepts modular coextrusion inserts was used with an insert machined for29-layer coextrusion. The die had an orifice width of 12 inches (30.48cm), and was maintained at about 282° C. Extrudates were cast onto achilled roll maintained at about 22° C. for the purpose of quenching thecast webs to a solid amorphous state. The quenched cast webs were about12-13 mils thick.

                  TABLE 1                                                         ______________________________________                                                   PEN IV  PET IV    "Surface"                                        Example No.                                                                              (dL/g)  (dL/g)    Polymer                                                                              % PEN                                     ______________________________________                                         1         0.57    --        All-PEN                                                                              100                                                                    Control                                           2         0.57    0.80      PET    80                                         3         0.57    0.80      PET    71                                         4         0.57    0.80      PET    59                                         5         0.57    0.80      PET    49                                         6         0.57    0.80      PET    41                                         7         0.57    0.80      PET    31                                         8         0.57    0.80      PET    20                                         9         --      0.80      All-PET                                                                               0                                                                     Control                                          10         0.50    --        All-PEN                                                                              100                                                                    Control                                          11         0.50    0.72      PET    70                                        12         0.50    0.72      PET    59                                        13         0.50    0.72      PET    49                                        14         0.50    0.72      PET    39                                        15         0.50    0.72      PET    30                                        16         0.50    0.72      PET    16                                        17         --      0.72      All-PET                                                                               0                                                                     Control                                          18         0.50    0.95      PEN    71                                        19         0.50    0.95      PEN    60                                        20         0.50    0.95      PEN    49                                        21         0.50    0.95      PEN    41                                        22         0.50    0.95      PEN    29                                        23         0.50    0.95      PEN    20                                        24         --      0.95      All-PET                                                                               0                                                                     Control                                          ______________________________________                                    

EXAMPLES 25-35

The following examples demonstrate the enhancement in modulus andstretch ratios of the multilayer films of the present invention incomparison with monolayer PEN.

The cast webs made in Examples 1-2 above were stretched into films usinga laboratory biaxial film stretching device. The stretching device was acustom-built instruments using a pantograph mechanism similar to thatfound in commercial instruments of its kind, such as the film stretchersavailable from T. M. Long Co. A square specimen of the cast web wasmarked with a gridline pattern and then mounted inside the filmstretcher, with the temperature inside the stretcher at or just below100° C. The temperature was quickly raised to 150° C. and the sample washeld for 45 seconds, measured from the beginning of the temperaturerise. The sample was then stretched simultaneously and equally in themachine and transverse directions at a rate of 100%/s, based on theoriginal gauge length of the sample. The gauge length is defined as thedistance between opposing pairs of grippers, as measured between theirclosest points. The stretching chamber was then opened and the samplewas quenched by blowing cool air across its surface and was thenremoved.

Stretch ratios for stretched samples were determined as the nominalstretch ratio and the real stretch ratio. "Nominal stretch ratio" refersto the final sample length divided by the gauge length, as determined bygrip separation. "Real stretch ratio" refers to the analogous figure, asmeasured by displacement of the central marks of the gridline patternwhich had been printed on the sample. As used throughout thisspecification, the phrase "biaxial stretch ratio" refers to the nominalstretch ratio (in each direction) for a simultaneous stretch of equalmagnitude in each direction. Real stretch ratios and modulus valuesreported without reference to machine or transverse directions areaveraged values for the two directions.

Specimens were prepared from the cast webs made in Examples 1 (100% PEN)and 2 (20% PET, 80% PEN). These specimens were stretched to variousbiaxial stretch ratios, until a stretch ratio was found at which it wasdifficult to stretch without specimen failure. The resulting stretchedfilms were tensile tested to determine their Young's Moduli. The resultsof these stretching experiments are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                Cast Web          Nominal                                                                              Real  Modulus,                               Example from              Stretch                                                                              Stretch                                                                             kpsi                                   No.     Example No.                                                                             % PEN   Ratio  Ratio (10.sup.6  kPa)                        ______________________________________                                        25      1         100     3.50   3.74  858 (5.9)                              26      1         100     4.00   4.00  910 (6.27)                             27      1         100     4.50   4.41  982 (6.77)                             28      1         100     5.00   4.78  1043 (7.19)                            29      1         100     5.25   5.10  1078 (7.43)                            30      2         80      3.50   3.50  731 (5.04)                             31      2         80      4.00   3.89  835 (5.76)                             32      2         80      4.50   4.36  916 (6.32)                             33      2         80      5.00   4.70  995 (6.86)                             34      2         80      5.50   5.19  1066 (7.35)                            35      2         80      5.75   5.51  1181 (8.14)                            ______________________________________                                    

These results are depicted graphically in FIG. 2. FIG. 2 demonstratesthat each composition develops a monotonically increasing Young'sModulus as the simultaneous biaxial stretch ratio is increased. At anygiven stretch ratio not resulting in sample failure, PEN shows a highermodulus than the 20:80 PET:PEN multilayer film, a result that might beexpected in light of the fact that PEN is known to be a higher moduluspolymer than PET. However, the multilayer cast web is unexpectedlycapable of being stretched to a considerably higher stretch ratiowithout sample failure as compared to monolithic PEN. Consequently, themodulus of the multilayer film is seen to ultimately surpass that of thePEN film, which is stretchable only to a lower stretch ratio.

EXAMPLES 36-44

The following examples demonstrate the effect of the PEN:PET Ratio onstretchability and modulus.

Experiments were performed to determine the highest stretch ratio towhich the cast webs of Examples 1-9 could be stretched at the conditionsof Examples 25-35. The breaking of a film during stretching is astatistical event, so that different specimens cut from a given cast webwill stretch to varying extents before breaking. For the purpose ofthese examples, the stretch ratio was examined at increments of 0.25nominal stretch ratio units until a ratio was found at which the samplebroke during stretching. This condition was repeated until threeconsecutive sample failures were recorded, or until two samplesstretched without breaking. The highest value of stretch ratio to whicha stretching experiment could be completed and replicated withoutspecimen rupture is called the Ultimate Biaxial Stretch Ratio (UBSR).Corresponding Real Stretch Ratios were determined as in Examples 25-35,by the displacement of ink marks.

At the UBSR for each composition, specimens were tensile tested todetermine their Young's Moduli. Some of these films were also mountedunder restraint on metal frames, and heat-set in an oven. The oven wasallowed to equilibrate at 235° C., the door was quickly opened, theframed specimen inserted, and the door immediately closed. The specimenwas left in the oven for 30 seconds and then removed. These heat-setspecimens were also tensile tested for Young's Modulus. The UBSR,Modulus, and Heat-set Modulus results are shown in tabular form in Table3 and graphically in FIGS. 3 and 4.

                  TABLE 3                                                         ______________________________________                                               Cast Web                           Heat-Set                                   from                        Modulus,                                                                             Modulus,                            Example                                                                              Example  %      UBSR  UBSR  kpsi   kpsi                                No.    No.      PEN    (nom) (real)                                                                              (10.sup.6 kPa)                                                                       (10.sup.6 kPa)                      ______________________________________                                        36     1        100    5.25  5.10  1078   1178                                                                   (7.43) (8.12)                              37     2        80     5.75  5.51  1181   1304                                                                   (8.14) (8.99)                              38     3        71     5.75  5.46  1071   1197                                                                   (7.38) (8.25)                              39     4        59     5.25  5.00  1005   1124                                                                   (6.93) (7.75)                              40     5        49     5.00  4.61   948   1047                                                                   (6.54) (7.22)                              41     6        41     4.25  3.88   811   --                                                                     (5.59)                                     42     7        31     3.50  3.06   648   --                                                                     (4.47)                                     43     8        20     3.25  2.86   556   --                                                                     (3.83)                                     44     9         0     3.00  2.07   443   --                                                                     (3.05)                                     ______________________________________                                    

As shown in Table 3 and FIG. 3, the UBSR varies smoothly withcomposition for the cast webs of Examples 1-9, with a maximum value neara composition of 70 to 80% PEN. For multilayer specimens consisting ofat least about 60% PEN, these values are about as high, or higher, thanthose observed with samples consisting of 100% PEN. Since PET itself isknown generally to be less stretchable than PEN, it is an unexpectedresult that the multilayer films of the two polymers should stretch tohigher ratios than either polymer alone.

Table 3 and FIG. 4 clearly show that the dependence of the modulus onthe composition, when measured at the UBSR, follows the same generalshape, that the modulus is highest near a composition of 80% PEN, andthat any of these multilayer compositions having at least about 70% PENis capable of having a modulus equal to or greater than that of 100%PEN. Since PET is known generally to be a polymer of lower modulus thanPEN, it is particularly unexpected that the multilayer films of the twopolymers should have Young's Moduli higher than those of either PEN orPET alone Table 3 and FIG. 4 also illustrate the effect of heat-settingin improving the modulus of any of the films of this invention.

EXAMPLES 45-57

The following examples illustrate the linear dependence of the modulusof the multilayer compositions of the present invention on (% PEN) andthe real stretch ratio.

Additional specimens were prepared from the cast webs of Examples 3-6.These were stretched to biaxial stretch ratios of 3.5 or higher, andtheir moduli were determined as before. The results are shown in Table4. The data from Examples 25-57 were pooled and fitted to a mathematicalmodel, assuming that the modulus depends linearly on both thecomposition (% PEN) and the real stretch ratio.

                  TABLE 4                                                         ______________________________________                                                Cast Web of       Stretch                                                                             Stretch                                                                             Modulus,                                Example Example           Ratio Ratio kpsi                                    No.     No.       % PEN   (nom) (real)                                                                              (10.sup.6  kPa)                         ______________________________________                                        45      3         71      3.50  3.39  741 (5.11)                              46      3         71      4.00  3.97  824 (5.68)                              47      3         71      4.50  4.31  903 (6.23)                              48      3         71      5.00  4.72  992 (6.84)                              49      3         71      5.50  5.14  1034 (7.13)                             50      4         59      4.00  3.80  787 (5.43)                              51      4         59      4.50  4.22  886 (6.11)                              52      4         59      5.00  4.74  956 (6.59)                              53      5         49      3.50  3.30  727 (5.01)                              54      5         49      4.00  3.68  804 (5.54)                              55      5         49      4.50  4.20  872 (6.01)                              56      6         41      3.50  3.22  707 (4.87)                              57      6         41      4.00  3.68  747 (5.15)                              ______________________________________                                    

The result of the mathematical fit is shown graphically in FIGS. 5 and6. It is immediately apparent that the data is well-fit by a linearmodel. The model also yields reasonable values for several limitingcases. Thus, FIG. 5 shows that the model predicts a modulus for pure PETbiaxially oriented to a stretch ratio of 4.0 that is roughly 760 kpsi(5.24×10⁶ kPa). This value is comparable to those observed with PETfilms made by conventional industrial processes. The model also predictsa modulus for pure PEN biaxially oriented to a stretch ratio of 5.0 thatis roughly 1070 kpsi (7.38×10⁶ kPa), which is comparable to the valuesobserved with commercially available PEN films. FIG. 6, which shows awider view of the same model, shows that the modulus values at stretchratio of 1.0 are roughly 260 kpsi (1.79×10⁶ kPa) and 350 kpsi (2.41×10⁶kPa) for PET and PEN, respectively. These values also compare reasonablywith those observed for pure samples of the polymers in question intheir unstretched states.

These results imply that the assumptions of the model are reasonable,and that the extrapolations of the other lines of constant stretch ratioin FIG. 6 are also significant. This suggests that the contribution ofthe PET layers to the overall modulus of the multilayer films stretchedto stretch ratios of 5.5 is slightly in excess of 1000 kpsi (6.9×10⁶kPa). It must be noted that a monolayer free-standing film of PETtypically cannot be stretched to stretch ratios as high as 5.5 in eachdirection by known commercial processes, and that the modulus of PETfilm made by such processes does not reach values in excess of 1000 kpsi(6.9×10⁶ kPa) in each direction.

Therefore, the results obtained in these examples, and the success ofthe linear model in predicting the observed results, imply that the PETlayers within the multilayer films are stretchable to much higher drawratios than can be achieved in conventional processes, and possessmoduli far in excess of those attainable with conventional PET films. APET-layer "contribution" to the overall film modulus of over 1000 kpsi(6.9×10⁶ kPa) is a particularly surprising result, as is thestretchability of PET layers to stretch ratios of 5.5.

EXAMPLES 58-61

The following examples demonstrate the dimensional stability of thefilms of the present invention.

Multilayer film samples from cast webs 1, 2, 3, and 9 were prepared bystretching, simultaneously and equally in both directions, on thelaboratory film stretcher. Conditions are given in Table 5. The stretchratios chosen for each cast web were at or near the UBSR for the chosenstretch temperatures. The films were heat-set on frames as in Examples36-40. The CTE, CHE, and 80° C./3 day shrinkage were measured onspecimens cut on the diagonal, so as to average the effects of the twodirections. The results are presented in Table 5.

                  TABLE 5                                                         ______________________________________                                        Exam- Cast           Stretch                                                                             Biaxial                                                                             CTE  CHE                                     ple   Web    %       Temp. Stretch                                                                             (ppm/                                                                              (ppm/ Shrinkage                         No.   No.    PEN     (°C.)                                                                        Ratio °C.)                                                                        % RH) (%)                               ______________________________________                                        58    9      PET     100    3.75 17.74                                                                              10.05 0.38                                           Control                                                          59    1      PEN     150   5.0    6.13                                                                               9.83 0.15                                           Control                                                          60    2      80      150   6.0    4.68                                                                               9.25 0.20                              61    3      71      150   5.5    3.97                                                                               9.02 0.21                              ______________________________________                                    

The results clearly reflect the well-known superior dimensionalstability of PEN over PET. Moreover, the results also show that themultilayer films exhibit somewhat improved CTE and CHE values over eventhe pure PEN film, and shrinkage values roughly equivalent to that whichwould be obtained from an interpolation based on composition between thevalues of the PET and PEN films.

EXAMPLES 62-88

The following examples illustrate the effect of temperature onstretchability and modulus.

Stretching experiments were performed on specimens of the cast web ofExample 2 to determine the effect of temperature on stretchability andthe resulting modulus. The procedures followed were similar to those ofExamples 36-44 above, except that the temperature was varied from 150°C. UBSRs were determined at temperatures from 120° to 180° C. In theseExamples, the UBSR is expressed only in terms of the nominal stretchratio to save the effort of measuring Real stretch ratios. Also, inthese Examples, a stretch ratio condition was pursued until fiveconsecutive sample failures were recorded (rather than three). Thus, thevalues reported for UBSR will be slightly higher if compared to those inExamples 36-44.

The laboratory stretcher used was capable of a maximum stretch ratioonly slightly excess of 6.0. At temperatures from 155° to 175° C., theUBSR was found to be in excess of 6.0, as evidenced by the lack ofruptured specimens when stretched to this extent. Therefore, in order tomore fully gauge the temperature effect, the somewhat less stretchablecast web of Example 5 was also tested.

The Young's Modulus of each film stretched to its UBSR was determined bytensile testing. The results are shown in Table 6 and in FIGS. 7-8. Itwas observed that all the films had a patchy or broken "frosted" or hazyappearance on each surface.

                  TABLE 6                                                         ______________________________________                                              Cast Web         Stretch         Modulus at                             Exam- of Exam-         Temper-         UBSR, kpsi                             ple No.                                                                             ple No.  % PEN   ature, °C.                                                                    UBSR     (10.sup.6  kPa)                        ______________________________________                                        62    2        80      120    4.00     632 (4.36)                             63    2        80      125    4.50     665 (4.59)                             64    2        80      130    4.50     799 (5.51)                             65    2        80      135    4.75     885 (6.10)                             66    2        80      140    5.00     931 (6.42)                             67    2        80      145    5.50     968 (6.67)                             68    2        80      150    6.00     1028 (7.09)                            69    2        80      155    >6.00    --                                     70    2        80      160    >6.00    --                                     71    2        80      165    >6.00    --                                     72    2        80      170    >6.00    --                                     73    2        80      175    >6.00    --                                     74    2        80      180    Unstretchable                                                                          --                                     75    5        49      120    3.75     --                                     76    5        49      125    4.25     --                                     77    5        49      130    4.25     726 (5.01)                             78    5        49      135    4.50     799 (5.51)                             79    5        49      140    4.50     774 (5.34)                             80    5        49      145    4.75     807 (5.56)                             81    5        49      150    4.75     864 (5.96)                             82    5        49      155    5.00     886 (6.11)                             83    5        49      160    5.25     861 (5.94)                             84    5        49      165    5.50     --                                     85    5        49      170    5.50     664 (4.58)                             86    5        49      175    5.25     --                                     87    5        49      180    5.25     --                                     88    5        49      185    4.75     --                                     ______________________________________                                    

FIG. 7 shows that the UBSR for the 80% PEN multilayer achieves a maximumat a temperature somewhere between 150° and 180° C., falling off sharplyat the high-temperature end of the range. The UBSR also appears to falloff more abruptly as the stretch temperature is lowered below 125° C.,which is very near the Tg of PEN. The 49% PEN composition exhibits asimilarly dependence of UBSR on stretch temperature, although the UBSRfalls off more gradually at very high temperatures as compared to the80% PEN composition.

This is effect may be due in part to the crystallization of the PETbefore the stretching commences at these high temperatures. Generally,170°-180° C. is regarded as the temperature range in which PETcrystallizes from the amorphous glass most rapidly. With PET making upmore of the total in the 49% PEN composition, the sample may be betterable to support drawing stresses at the higher temperatures. It is alsoapparent that the 49% PEN composition has a maximum UBSR at 165°-170° C.

As indicated in FIG. 8, the modulus at the UBSR for the 80% PENcomposition rises with stretch temperature up to the point where machinelimitations make further measurements impossible. The modulus of thefilm made at 150° C. was in excess of 1000 kpsi (6.9×10⁶ kPa) prior toheat-setting, and the curve of modulus as a function of stretchtemperature shows no signs of leveling off. The results for the 49% PENcomposition, however, show a maximum at a stretch temperature somewhatlower than that of the UBSR maximum. Thus, the optimum stretchingtemperature range for the 80% PEN composition is also likely to be inthe 150°-160° C. range. Since the glass transition of PEN is only about120°-125° C. and the glass transition of PET is much lower, thedetermination of an optimum stretching temperature of 150°-160° C. forthe multilayer films is a surprising result.

EXAMPLES 89-103

The following examples illustrate the application of the feedblockconcept of multilayer coextrusion for the PEN:PET polymer pair.

Samples of PEN and PET were obtained and were dried under dry nitrogen,PEN at about 177° C., and PET at about 149° C. The PEN resins used hadseveral different molecular weights, as measured by intrinsic viscosity(IV). The PET resin was Goodyear Traytuf 8000C, with an IV of 0.80. ForPEN, a 13/4 inch extruder was used, and the extrusion temperature wasabout 293° C. For PET, a second 1.75 inch (4.4 cm) extruder was used,and the extrusion temperature was about 282° C.

The resins were coextruded by a feedblock method. Thus, the melt streamsfrom the two extruders were conveyed to the feedblock via 3/4" diameterneck tubes maintained at about 293° C. and 266° C., respectively, forPEN and PET. A modular feedblock with an alternating-two-component,29-layer insert was used. The feedblock fed a typical polyester film diewith a 12 inch (30.5 cm) wide die orifice. The feedblock exit was matedto the die inlet via a gradual square-to-round flow channel profileadapter.

The feedblock, adapter, and die were all maintained at about 282° C. Theextrudate was cast onto a chill roll maintained at about 18° C., andelectrostatic pinning was used. Total combined throughput was maintainedat either about 60 lbs/hr (7.5×10⁻³ kg/s) or 90 lbs/hr (1.1×10⁻² kg/s).The PEN:PET ratio was varied from about 80:20 to about 50:50. Thefeedblock was set up so that the outermost layers were PET in someexperiments and PEN in others. The cast web thickness was controlled bythe chill roll speed to be about 12-13 mils. In some experiments, the2nd and 28th slots of the feedblock were plugged, so as to create a25-layer flow with outermost layers of double thickness.

The cast films were evaluated before any stretching for characteristicrheologically-based flow-defect patterns, and rated "Good", "Marginal",or "Poor". "Good" cast webs exhibited no flow-defect patterns,"Marginal" webs exhibited minor cosmetic flow-defect patterns, and"Poor" webs exhibited significant flow-defect patters Table 7 containsthe conditions of the individual experiments and results of theevaluations.

                  TABLE 7                                                         ______________________________________                                        Exam-         PEN    Throughput,    Outside                                   ple   No. of  IV,    lbs/hr  PEN:PET                                                                              Layer Cast Web                            Number                                                                              Layers  dL/g   (10.sup.-3 kg/s)                                                                      Ratio  Polymer                                                                             Rating                              ______________________________________                                        89    29      0.626  63 (7.9)                                                                              80     PET   Poor                                90    29      0.570  59 (7.4)                                                                              80     PET   Poor                                91    29      0.520  61 (7.7)                                                                              81     PET   Poor                                92    29      0.473  61  7.7)                                                                              80     PET   Good                                93    29      0.473  62 (7.8)                                                                              70     PET   Good                                94    29      0.473  62 (7.8)                                                                              61     PET   Good                                95    29      0.473  61 (7.7)                                                                              53     PET   Marginal                            96    25      0.570  60 (7.6)                                                                              79     PET   Poor                                97    25      0.516  59 (7.4)                                                                              80     PET   Marginal                            98    25      0.516   94 (11.8)                                                                            79     PET   Marginal                            99    25      0.485  63 (7.9)                                                                              80     PET   Good                                100   25      0.485   93 (11.7)                                                                            80     PET   Good                                101   25      0.555  61 (7.7)                                                                              79     PEN   Poor                                102   25      0.516  59 (7.4)                                                                              79     PEN   Marginal                            103   25      0.485  60 (7.6)                                                                              78     PEN   Good                                ______________________________________                                    

These results indicate that, with the feedblock configuration used, itwas necessary to utilize a PEN resin with IV below 0.52 in order to makeacceptable multilayer cast webs with a PET resin of IV 0.80, regardlessof which polymer was used on the surface layers. The same feedblock anddie were used in subsequent experiments on continuous film lines. Sincethe mechanical properties of PEN decrease with an IV below a level ofabout 0.53, comparison of properties between prior subsequent examplesmay be misleading.

EXAMPLES 104-105

The following examples illustrate the effect of IV on stretchability.

Specimens were prepared for stretching experiments from the cast webs ofExample 3 (for Example 104) and Example 11 (for Example 105). These castwebs were chosen because the only significant difference between themwas the IV of the resins used. The cast web of Example 3 consisted ofPEN with IV of 0.57 and PET with IV of 0.80. The cast web of Example 11consisted of PEN with IV of 0.50 and PET with IV of 0.72. Each cast webhad PET at the outermost layers, and consisted of about 70% PEN.

For each cast web, the UBSR was determined as in Examples 50-76, at 150°C. In Example 104, the UBSR was determined to be 5.75. In Example 105, avalue of 5.25 to 5.50 was obtained. Thus, the higher IV resins appear topromote the enhanced stretchability effect.

EXAMPLES 106-111

The following examples illustrate the effect of cast web quality onstretchability.

Specimens were prepared for stretching experiments from the cast webs ofExample 2 (for Example 106) and Example 90 (for Example 107). These castwebs were chosen because the only significant difference between themwas that the web from Example 2 was prepared using the multilayer die,while the web from Example 90 was prepared using the less Theologically"forgiving" multilayer feedblock. Thus, the web from Example 90 includedrheologically-related surface imperfections, as reflected by its castweb rating of "poor" in Table 7. Each cast web consisted of 80% PEN andhad PET as the outermost layers. The resins used in the web also hadsimilar IVs.

For each cast web, the UBSR was determined as in Examples 62-88, at 150°C. In Example 106, the UBSR was determined to be 6.00, the stretchingmachine's physical limit. In Example 107, a USBR of 5.25 was obtained.Thus, the rheologically-related defects appear to negatively impact theenhanced stretchability of the films.

Specimens were prepared for stretching experiments from the cast webs ofExample 91 (for Example 108) and Example 92 (for Example 109). Thesecast webs were chosen because, taken with the cast web of Example 90(Example 107), they constitute a series in which the only significantdifferences are the IVs of the PEN resins used, and consequently, thequality of the cast web surface. The cast web of Example 90 containedPEN with an IV of 0.570, and was rated "poor" in surface quality due torheologically-related defects. The cast web of Example 91 contained PENwith an IV of 0.520, and was also rated "poor" in surface quality. Thecast web of Example 92 contained PEN with an IV of 0.473, and was rated"good" in surface quality. Each cast web had PET as the outermostlayers, and consisted of about 80% PEN.

For each cast web, the UBSR was determined as described in Examples62-88 at 150° C. In Example 107 , the UBSR was 5.25, as stated above. InExample 108, a value of 5.75 was obtained. In Example 109, a value of6.00 (stretching machine limit) was obtained. Since the effect of resinIV shown by Examples 104-105 would predict UBSRs falling in the reverseof this order, the surface quality is shown by these Examples to be aneven more important factor in promoting enhanced stretchability in themultilayer films.

Specimens were prepared for stretching experiments from the cast webs ofExample 96 (for Example 110 ) and Example 99 (for Example 111 ). Thesecast webs were chosen because the only significant differences betweenthem are the IVs of the PEN resin used, and consequently, the quality ofthe cast web surface. Together, they differ from the Examples 107-109series in having 25 alternating layers, with the outermost layersdouble-thick, rather than 29 alternating layers of equal thicknesses.

The cast web of Example 96 contained PEN with IV of 0.570, and was rated"poor" in surface quality due to flow-related defects. The cast web ofExample 99 contained PEN with IV of 0.485, and was rated "good" insurface quality. Each cast web had PET at the outermost layers, andconsisted of about 80% PEN. For each cast web, the UBSR was determinedas described in Examples 62-88 at 150° C. In Example 110, the UBSR was5.50. In Example 111 , a value of 6.00 (stretching machine limit) wasobtained. Clearly, the deleterious effect on stretchability demonstratedby Examples 107-109 is shown to continue to apply to these films, eventhough they were made with doubly-thick surface layers.

The results of Examples 107 and 110 were further compared. The higherUBSR in the case of Example 110 (5.50 vs. 5.25) suggests that there is abeneficial stretchability effect, of secondary importance, from theprovision of doubly-thick surface layers on the multilayer films.

EXAMPLES 112-113

The following examples illustrate the effect of the PEN IV on themodulus.

The modulus was determined for the films stretched to their 150° C. UBSRin Examples 108 and 109 (Examples 112 and 113, respectively). In Example112, the modulus was found to be 1000 kpsi (6.90×10⁶ kPa) at a biaxialstretch ratio of 5.75. For Example 113, the modulus was determined to be946 kpsi (6.52×10⁶ kPa) at a biaxial stretch ratio of 6.00. The higherIV PEN resin appears to be beneficial in promoting a higher modulus, inthis case even overcoming a disadvantage in stretchability.

EXAMPLES 114-117

The following examples demonstrate the effect of the choice of surfacepolymer and the degree of crystallinity of PET on the clarity andfrictional properties of multilayer PEN/PET films. The examples alsoillustrate the behavior of films in which the PET layers are"constrained".

Specimens for Examples 114-117 were prepared from the cast webs ofExamples 1 (Monolayer PEN), 3 (71% PEN with PET as the "surface"polymer), 18 (71% PEN with PEN as the "surface" polymer), and 9(monolayer PET), respectively. The first three specimens were stretchedat conditions similar to Examples 25-35, to biaxial stretch ratios of5.0 at a stretch temperature of 150° C. The fourth, being pure PET, wasmounted in the stretcher at 60° C., and stretched at 100° C. to biaxialstretch ratio of 4.0. Examples No. 114 (PEN), No. 116 (71% PEN with PENas "surface" polymer), and No. 117 (PET) each yielded visually clear,non-hazy films, while Example No. 115 (71% PEN with PET as "surface"polymer) yielded films with a patchy haze as in Examples 62-88. All ofthe multilayer films, even those referred to as being "clear", exhibiteda slightly iridescent appearance, most likely due to the proximity ofthe individual layer thicknesses of the stretched films to thewavelengths of visible light.

Specimens of Example No. 115 were also observed to be slippery whenfolded over and rubbed against themselves. By contrast, the PEN and PETfilms (Examples Nos. 114 and 117) "block" to themselves tenaciously andare very hard to slide in friction. Surprisingly, the multilayer filmwith the PEN outer layers (Example No. 116) exhibited frictionalbehavior intermediate between these two extremes.

Without wishing to be bound by any theory, it is believed that in thecase of the multilayer films, the elevated temperature of 150° C.required for stretching the PEN causes the PET layers to crystallizeduring preheating, prior to the commencement of stretching. In the caseof films with PET as the outermost layers, the crystallized PET surfacelayers are believed to break up during the stretching step, leaving"islands" of patchy haze on the stretched film. Surprisingly, when PENserves as the outermost layers, no patchiness or haziness is observed.It is believed that the PET layers still crystallize during preheat, butthat the PET draws without failure from the crystalline state whenconfined between the PEN layers.

EXAMPLES 118-121

The following examples illustrate the effect of the surface polymer onstretchability and modulus.

Specimens were prepared for stretching experiments from the cast webs ofExample 99 (for Examples 118 and 120) and Example 103 (for Examples 119and 121). These cast webs were chosen in light of the fact that the onlysignificant difference between them was the identity of the polymer inthe two outside surface layers. The cast web of Example 99 had 25 layerswith PET forming both outside or surface layers, while the cast web ofExample 103 had 25 layers with PEN forming both surface layers. Eachspecimen consisted of about 80% PEN.

For each cast web, the UBSR was determined as described in Examples62-88 at both 150° and 145° C. The Examples done at 145° C. wereperformed for the sake of resolving a stretchability difference betweenthe two cast webs, since both proved stretchable to the machine limit at150° C. For the films drawn to the same nominal draw ratio at 150° C.,the real draw ratio was determined by the displacement of ink marks. Themodulus was also determined. Both are reported as values averaged overthe MD and TD. The results are shown in Table 8.

                  TABLE 8                                                         ______________________________________                                                                     Ultimate                                                Cast            Stretch                                                                             Biaxial                                                                              Real  Modulus,                            Exam-  Web    "Outside"                                                                              Temp. Stretch                                                                              Stretch                                                                             kpsi                                ple No.                                                                              No.    Polymer  (°C.)                                                                        Ratio  Ratio (10.sup.6  kPa)                     ______________________________________                                        118     99    PET      145   5.25                                             119    103    PEN      145   5.50                                             120     99    PET      150   >6.00  5.70  1018                                                                          (7.02)                              121    103    PEN      150   >6.00  5.89  1037                                                                          (7.15)                              ______________________________________                                    

These results demonstrate that the stretching differences betweenotherwise identical cast webs, due solely to the choice of surface-layerpolymer, are small. PEN surface layers appear to promote slightlyenhanced stretchability, a more uniform draw (i.e., a real stretch ratiocloser to the nominal value), and a slightly higher modulus. As inExamples 114-117, the films with PEN outer layers were also clear, whilethe PET-surfaced films had uneven patches of frosty haze.

The placement of the lower-Tg PET at the surface layers presents somepractical challenges in a continuous process, especially in a lengthorienter or tenter, where the film is contacted across its width or atthe edges by metal parts heated to a temperature sufficiently high forstretching the higher-Tg PEN. Since the results of these Examples showno advantage to placing the PET at the surface layers, all subsequentExamples employ "PEN-surfaced" constructions.

EXAMPLES 122-124

The following examples demonstrate the production of the film of thecurrent invention in a continuous manner on a film line.

A PEN resin was prepared having an IV of 0.50, and was dried at about149° C. A PET resin (Goodyear Traytuf 8000C) was obtained which had anIV of 0.80, and was dried at about 135° C. The PEN was extruded on a21/2" single screw extruder at a temperature of about 293° C., with thepost-extruder equipment in the PEN melt train being maintained at about282° C. The PET was extruded on a 13/4" single screw extruder at atemperature of about 277° C., with the post-extruder equipment in thePET melt train being maintained at about 266° C. Gear pumps were used tocontrol the extrudate flow. Both melt streams were filtered withcandle-type filters rated for 40 microns, and 3/4-inch diameter, heated,insulated neck tubes were used to convey the polymer melts to thefeedblock.

The same feedblock insert was used as in Examples 89-103, and wasplugged as before to give a 25-layer construction whose outermost layerswere doubled in thickness. The feedblock was fed to place PEN as theoutermost layers. The PEN:PET ratio was 80:20 by weight, and totalthroughput was about 130 lbs/hr. The same 12" wide film die as inExamples 89-103 was used. Electrostatic pinning was also used. Thefeedblock was maintained at a temperature of about 282° C, and the diewas maintained at a temperature of about 288° C. The casting roll wasmaintained at a temperature of about 52° C. The casting roll speed wasadjusted to provide a cast web thickness of 12 to 13 mils.

Using a "length orienter", the cast web was stretched in the machinedirection between rolls driven at different speeds. The slower drivenrolls were maintained at about 138° C. and subsequent idler rolls weremaintained at about 143° C. The nominal stretch ratio in this step,determined by the difference in speeds of the driven rolls, was 1.30.The faster (cooling) rolls were maintained at about 24° C.

The film was subsequently stretched in both the machine and transversedirections using a tenter capable of simultaneous biaxial stretching.The tenter oven's preheat and stretch zones were both maintained atabout 163° C. The preheat zone had a length of 9.8 feet (3.0 m),providing a residence time in the preheat zone of approximately 18seconds at those conditions. The film was further stretched nominally(as measured by grip displacement) to stretch ratios of 4.40 and 4.89 inthe machine and transverse directions, respectively. The stretch zonehad a length of 8.2 feet (2.5 m), providing a residence time in thestretch zone of approximately 6 seconds at those conditions.

The film was heat-set under restraint in the tenter. The tenter's twoheat-set zones were maintained at about 216° and 199° C. Before releasefrom the tenter clips, the film was cooled in a cooling zone maintainedat about 54° C. Ink marks were drawn on the cast web in order to measurethe actual stretch ratios in the center of the film web. The finalstretch ratios were 5.81 and 5.50 in the machine and transversedirections, respectively. The film was, surprisingly, somewhat hazy, inspite of having PEN outer layers. In addition, rather than beingslightly and uniformly iridescent over its entire surface, as wasobserved of almost all of the lab stretcher specimens of multilayerfilms, the film of this Example had lightly colored bands running in themachine direction, probably due to minor thickness and/or orientationaldifferences cross-web. The physical properties of the film of Example122 are listed in Table 9.

In Example 123, the length orienter's fast roll was adjusted to providea draw ratio of 1.34. The tenter's nominal draw ratios in the machineand transverse directions were 4.40 and 5.12, respectively. All otherconditions were unchanged. The stretch ratios of the finished film, asmeasured by the displacement of ink marks, were 5.99 and 5.95 in themachine and transverse directions, respectively. This film was equallyhazy and color-banded. The physical properties of the film are listed inTable 9.

In Example 124, the temperatures in the simultaneous-biaxial tenter werealtered. Other conditions were as before. At tenter preheat and stretchtemperatures of about 168° C. and 149° C., respectively, measuredstretch ratios of 6.14 and 6.11 were obtained in the machine andtransverse directions, respectively. This film was less hazy than thetwo described above. The physical properties of this film are listed inTable 9.

                  TABLE 9                                                         ______________________________________                                                            Example No.                                                                   122   123     124                                         ______________________________________                                        L.O. Stretch Ratio        1.30    1.34  1.34                                  Tenter Preheat Temp.                                                                          °C.                                                                              163     163   168                                   Tenter Stretch Temp.                                                                          °C.                                                                              163     163   149                                   Tenter MD Stretch Ratio   4.40    4.40  4.40                                  Tenter TD Stretch Ratio   4.89    5.12  5.12                                  Film Caliper    mils      0.363   0.340 0.306                                 Real Stretch Ratio (MD)   5.81    5.99  6.14                                  Real Stretch Ratio (TD)   5.50    5.95  6.11                                  Green Modulus (MD)                                                                            kpsi      890     792   760                                                   (10.sup.6 (6.14)                                              Green Modulus (TD)                                                                            kpsi      906     925   898                                                   (10.sup.6 (6.25)                                              Modulus (MD)    kpsi      966     1015  962                                                   (10.sup.6 (6.66)                                              Modulus (TD)    kpsi      1019    995   1078                                                  (10.sup.6 (7.03)                                              CTE (MD)        (ppm/°C.)                                                                        15.91   10.38 15.28                                 CTE (TD)        (ppm/°C.)                                                                        11.53   10.25 10.53                                 CHE (MD)        (ppm/%RH  11.03   9.53  8.78                                  CHE (TD)        (ppm/%RH) 8.82    8.67  7.43                                  65° C./72 hr. Shrinkage (MD)                                                           (%)       0.16    0.16  0.13                                  65° C./72 hr. Shrinkage (TD)                                                           (%)       0.18    0.17  0.17                                  150° C./15 min Shrinkage                                                               (%)       2.34    2.60  1.65                                  (MD)                                                                          150° C./15 min Shrinkage                                                               (%)       2.84    2.92  2.35                                  (TD)                                                                          Appearance                Hazy    Hazy  Less                                                                          Hazy                                  ______________________________________                                    

These results demonstrate that it is possible, by the process described,to produce the film of the current invention in a continuous manner on afilm line. However, the modulus values, being lower than those inExample 37, and the CTE values, being higher than those in Example 60,serve to illustrate that the conditions set forth in these threeexamples are not the optimum conditions, and that one skilled in the artmight reasonably expect to improve upon these properties via appropriateadjustment of the processing conditions.

EXAMPLE 125 AND COMPARATIVE EXAMPLES 1-3

The following examples illustrate the effect of the length orienter andtenter temperatures on the processability of the compositions of thepresent invention.

In Example 125, the length orienter was run with the heated rollsmaintained at about 149° and 154° C. At these conditions, the web tendedto develop a slack which could only be taken up by increasing the drawratio to 1.6 or more. Thus, film could not be successfully stretched tothe lower machine direction draw ratios of the earlier examples at theseconditions, but could be drawn to higher machine direction draw ratios.

In Comparative Example 1, the roll temperatures in the length orienterwere further increased to about 160°-166° C. At these conditions, theweb began to adhere to the rolls, and no stretched film could be made.

In Comparative Example 2, the temperatures of the preheat and stretchingzones of the tenter were maintained at about 177° C. At theseconditions, the web was blown apart by the turbulent air in the tenterand could not be stretched.

In Comparative Example 3, the temperatures of the preheat and stretchingzones of the tenter were maintained at about 149° C. At theseconditions, when attempting to stretch to draw ratios similar to thosein the above examples, the web tended to pull out of the grippers in thetenter, and could not be successfully stretched.

EXAMPLES 126-134

The following examples illustrate the effect of process parameters onthermal shrinkage of the films.

A series of Examples in the form of a designed experiment was preparedin order to search for conditions at which the irreversible thermalshrinkage might be decreased. Conditions were as in Example 122 above,with the following exceptions: PET resin was dried at about 132° C.Total throughput was about 100 lbs/hr (1.26×10⁻² kg/s) at 80% PEN byweight. The feedblock was maintained at about 282° C., and the die atabout 288° C. The temperature of the heating rolls on the lengthorienter were adjusted to improve their efficiency in heating the web,and were set at about 118° C. for the slower rolls and 124° C. for theidler rolls. The machine direction stretch ratio in the length orienterwas set to 1.35. Stretch ratios in the stretch zone of the tenter were4.40 in the machine direction and 4.62 in the transverse direction, asdetermined by grip separation.

In these Examples, three process parameters were varied: (1) thetemperature of the first heat-set zone (T_(HS1)); (2) the temperature ofthe second heat-set zone (T_(HS2)); and (3) the amount of relaxationallowed in the transverse direction by adjustment of the tenter rails.

The design of the tenter allows for the separation of the rails to benarrowed between the exit of the stretching zone and the exit of thetenter. The rails were adjusted so that the stretch ratio of the filmdecreased continuously as it traversed the heat-set zones. The"relaxation" parameter is expressed as the transverse direction stretchratio, determined by grip displacement, based on the positions at theentrance and exit to the tenter (SR_(REL)). Thus, low levels ofrelaxation are represented by values of SR_(REL) nearer to 4.62 (highervalues).

A 2-cubed factorial design with center point was performed. The low andhigh values for the three process parameters were as follows: T_(HS1:)193° and 216° C.; T_(HS2:) 193° and 216° C.; SR_(REL:) 4.49 and 4.23.The center point had values for the three parameters of 204° C., 204°C., and 4.36, respectively.

All films were about 0.35 mils in thickness. "Green" modulus wasdetermined by tensile test. Irreversible thermal shrinkage wasdetermined using the 150° C./15 min. test described previously. Each ofthese measurements was made in both the machine and transversedirections. Haze was also measured. Each value reported is the averageof two tests. The results are in Table 10.

                                      TABLE 10                                    __________________________________________________________________________                              150° C./                                                                    150° C./                                                Green                                                                              Green                                                                              15 min                                                                             15 min                                                         Mod. MD,                                                                           Mod. TD,                                                                           Shrinkage                                                                          Shrinkage                                      Example                                                                            T.sub.HS1                                                                         T.sub.HS2                                                                            kpsi kpsi MD,  TD,  Haze,                                     No.  °C.                                                                        °C.                                                                       SR.sub.REL                                                                        (10.sup.6 kPa)                                                                     (10.sup.6 kPa)                                                                     %    %    %                                         __________________________________________________________________________    126  204 204                                                                              4.36                                                                              721  728  1.95 0.50 10.30                                                     (4.97)                                                                             (5.02)                                                   127  216 216                                                                              4.49                                                                              668  771  1.70 1.00 12.70                                                     (4.61)                                                                             (5.32)                                                   128  216 193                                                                              4.49                                                                              710  770  1.55 1.45 8.55                                                      (4.90)                                                                             (5.31)                                                   129  193 193                                                                              4.49                                                                              746  820  2.75 2.00 7.70                                                      (5.14)                                                                             (5.65)                                                   130  193 216                                                                              4.49                                                                              775  799  1.00 0.95 6.70                                                      (5.34)                                                                             (5.51)                                                   131  193 216                                                                              4.23                                                                              777  740  0.85 0.25 9.05                                                      (5.36)                                                                             (5.10)                                                   132  216 216                                                                              4.23                                                                              753  721  1.05 0.10 8.75                                                      (5.19)                                                                             (4.97)                                                   133  216 193                                                                              4.23                                                                              739  740  1.50 -0.50.sup.1                                                                        8.90                                                      (5.10)                                                                             (5.10)                                                   134  193 193                                                                              4.23                                                                              739  767  2.65 0.35 14.80                                                     (5.10)                                                                             (5.29)                                                   __________________________________________________________________________     .sup.1 The negative value for Irreversible Thermal Shrinkage in the           transverse direction for Example 133 indicates that the sample actually       expanded irreversibly upon thermal treatment.                            

Standard statistical analyses of the design indicated that the measuredfilm properties affected to a statistically significant extent by thechanges in process conditions were transverse direction shrinkage,machine direction shrinkage, and transverse direction modulus, in orderof decreasing significance. Variations in haze and machine directionmodulus were statistically insignificant.

The effects on transverse direction shrinkage of Heat-Set Zone #1Temperature ("A"), Heat-Set Zone #2 Temperature ("B"), and Relaxation("C") were all statistically significant, as were the "AB" and "BC"interactions. The "AC" interaction is marginally significant.

The effects on machine direction shrinkage of "A" and "B" werestatistically significant, as was the "AB" interaction. The effect of"C" was not statistically significant.

The effects on the transverse direction modulus of "A" and "C" werehighly statistically significant, while the effect of "B" was ofmarginal significance. None of the interactions were significant.

Therefore, for transverse direction shrinkage, the highest level ofrelaxation is seen to result in general improvement, and a more precisedesired value for shrinkage can be achieved through adjustment of theheat-set temperatures. Zero shrinkage in the transverse direction isalso achievable. For machine direction shrinkage, the higher level ofheat-set zone #2 temperature results in general improvement, while theheat-set zone #1 temperature provides a means of additional control. Notsurprisingly, the transverse direction modulus benefits most from a lowlevel of relaxation, but a low temperature in heat-set zone #1 is alsobeneficial.

Thus, over the range studied, it was found that the combination of lowtemperature in heat-set zone #1, high temperature in heat-set zone #2,and a large relaxation result in the best overall control of shrinkagein both directions, with some loss of transverse direction modulus, butno statistically significant deleterious effects on any other measuredproperties.

EXAMPLES 135-137

The following examples illustrate the surface roughness of continuousprocess films having PEN in the outermost layers.

Upon testing, each of the films of Examples 122-124 was found to slidevery easily when folded over onto itself, in spite of having PEN ratherthan PET in the outermost layers. This was a very unexpected result, asit had not been observed in the laboratory-prepared film of Example 116,and since the films in question contained none of the particulate "slipagents" commonly used in the polyester film-making art to providefrictional "slip" properties. Because of this, measurements were made onthe surface roughness by both Interferometry and Rodenstock techniques.The static and kinetic coefficients of friction were also determined.These measurements are summarized in Examples 135-137 in Table 11.

EXAMPLES 138-141

The following examples illustrate the difference in the surfaceroughness and frictional behavior of the films made on the film line,compared to films made in the laboratory.

For comparison with Examples 135-137, specimens for laboratorystretching were prepared from cast webs of Example 1 (PEN), Example 103(78% PEN with PEN outermost layers), and Example 99 (80% PEN with PEToutermost layers). The specimens were stretched under the conditionsoutlined in Examples 25-35 to biaxial stretch ratios of 5.5, 6.0, and6.0, respectively, to give Examples 138-140.

An additional specimen of the cast web of Example 103 was stretched by atechnique intended to more closely model the film line conditions ofExamples 122-124. After the usual preheating at 150° C. for 45 seconds,the specimen was stretched in only the machine direction at a rate of100%/sec and a temperature of 150° C. to a stretch ratio of 1.364. Thespecimen was then immediately further stretched simultaneously in bothdirections to a stretch ratio in the transverse direction of 6.00, andan overall stretch ratio in the machine direction (based on the originalunstretched length) of 6.00. This required additional machine directionstretching in this step of 6.00/1.364, or 4.40 . The rate of transversedirection stretching was 100%/sec, and the rate of machine directionstretching was adjusted to cause the stretching in both directions toend simultaneously. There was no pause between the end of the machinedirection-only stretch and the commencement of the simultaneousstretching step. This film is Example 141.

The same analyses were performed as for Examples 135-137. The results ofthese analyses are set forth in Table 11. In the columns ofInterferometry and Rodenstock data, the two numbers represent the twosides of each film specimen.

                                      TABLE 11                                    __________________________________________________________________________                Outer                                                                             Interferometry                                                                       Interferometry                                                                       Rodenstock                                                                          Rodenstock                                Ex.                                                                              Stretch                                                                            %   Layer                                                                             Ra     Rq     Ra    Rq    Static                                                                           Kinetic                          No.                                                                              Method                                                                             PEN Polymer                                                                           (nm)   (nm)   (nm)  (nm)  COF                                                                              COF                              __________________________________________________________________________    135                                                                              Film 80  PEN 12.83  21.87  47    79    0.66                                                                             0.38                                Line         13.88  20.26  40    71                                        136                                                                              Film 80  PEN 9.06   10.47  39    63    0.80                                                                             0.48                                Line         11.51  17.93  34    57                                        137                                                                              Film 80  PEN 19.50  27.11  53    95    0.61                                                                             0.44                                Line         21.26  31.44  65    112                                       138                                                                              Lab  PEN PEN 3.29   3.92    8    10    3.20                                                                             off                                 Stretcher                                                                          Control 6.31   7.72    9    14       scale                            139                                                                              Lab  78  PEN 3.49   4.74   18    30    1.92                                                                             0.88                                Stretcher    5.53   6.75   16    21                                        140                                                                              Lab  80  PET off    off    134   234   0.35                                                                             0.29                                Stretcher    scale  scale  194   359                                       141                                                                              Lab  78  PEN 3.79   4.84   14    18    1.11                                                                             0.70                                Stretcher/   4.98   8.91   15    21                                           Line                                                                          Simulation                                                                 __________________________________________________________________________

The results depicted in Table 11 clearly show that there is anunexpected difference in the surface roughness and frictional behaviorof the films made on the film line, compared to films made in thelaboratory.

The PEN Control (Example 138) is, as would be expected for a polyesterfilm containing no added slip agent, quite smooth, and showsexceptionally high coefficients of friction. The PEN-surfaced multilayerfilm made in the laboratory (Example 139) is almost as smooth. Thedifference between the laboratory produced film and the PEN control ismost clearly seen in the Rodenstock numbers, which are not as sensitiveto long-range curvature of the specimen surface as are theInterferometry data at such low levels of surface roughness. Thecoefficients of friction are also somewhat lower, though still high. Bycontrast, the PET-surfaced multilayer film made in the laboratory(Example 140) shows exceptionally high surface roughness, as would beexpected from its frosted or hazy appearance, and correspondingly lowcoefficients of friction.

Surprisingly, the PEN-surfaced films made on the film line (Examples135-137) clearly show surface roughness and frictional propertiesintermediate between the laboratory films of similar composition and thePET-surfaced laboratory films. The stretch conditions of Example 141more closely simulate the film line conditions, but its surface andfrictional properties much more closely resemble those of the otherlaboratory-made film (Example 139) than the film line examples.

These differences can be more clearly seen in FIGS. 9-14, which show3-dimensional plots of the Interferometry data of Examples 135-139 and141, respectively. These figures indicate qualitatively that the PENControl film of Example 138 and FIG. 12 is clearly the smoothest,followed by the PEN-surfaced laboratory films of Examples 139 and 141and FIGS. 13 and 14, which closely resemble each other. The film linefilms of Examples 135-137 and FIGS. 9-11 are considerably rougher, andalso resemble each other qualitatively. Finally, the PET-surfaced filmof Ex. 140 is too rough to be measured by interferometry.

EXAMPLE 142

The following example illustrates the effect of casting on surfaceroughness.

Some of the cast web from the film line, made at the conditions outlinedin Example 122, was collected prior to the in-line stretching steps, andwas retained. In order to determine if the unusual surface roughnessobserved in the finished films was already present in the cast web, aspecimen was analyzed by interferometry. The Ra and Rq values were 4.49nm and 5.50 nm on one side and 4.89 nm and 6.53 nm on the other side. Itwas concluded that the high surface roughness was not attributable tothe film casting process.

EXAMPLES 143-146

The following examples illustrate the effect of length orientation onsurface roughness.

In order to confirm that the surface roughness was not caused directlyby the length orientation process, Rodenstock surface roughnessmeasurements were made on one specimen of film wound after the castingwheel with no stretching at all, and three specimens of film collectedafter the length orienter with no tenter stretching. Otherwise, lineconditions of Examples 126-134 were used. The results are shown in Table12:

                  TABLE 12                                                        ______________________________________                                                   T.sub.LO          Rodenstock Ra                                    Example No.                                                                              (°C.)                                                                             SR.sub.LO                                                                            (nm)                                             ______________________________________                                        143        none       none   19                                               144        116        1.34   18                                               145        121        1.34   15                                               146        138        1.34   15                                               ______________________________________                                    

Since the length-oriented films (Examples 144-146) are all smoother thanthe cast web (Example 143), it is confirmed that roughening of the filmoccurs within the tenter and is not related to the roughness of thelength-oriented web.

EXAMPLES 147-148

The following examples illustrate the effect of heat-setting on surfaceroughness.

In the preceding examples, none of the laboratory films examined forsurface roughness were heat-set. To explore the possibility that theunexpected surface roughness of the film line films of Examples 135-137was caused by the heat-setting step, two more specimens were preparedfor laboratory stretching from the cast web retained from the film lineExample 122. Simultaneous biaxial stretching experiments were performedat conditions similar to those of Examples 25-35, to a biaxial stretchratio of 5.75. One film sample (Example 147) was tested as made. Theother (Example 148) was heat set on a frame, using the heat-settingconditions of Examples 39-40, and was subsequently tested for surfaceroughness and COF. The results are shown in Table 13.

                                      TABLE 13                                    __________________________________________________________________________            Interferometry                                                                       Interferometry                                                                       Rodenstock                                                                          Rodenstock                                        Example                                                                            Heat-                                                                            Ra     Rq     Ra    Rq    Static                                                                           Kinetic                                  No.  Set?                                                                             (nm)   (nm)   (nm)  (nm)  COF                                                                              COF                                      __________________________________________________________________________    147  NO 3.18   4.04   16    22    4.04                                                                             off                                              4.28   5.23   18    26       scale                                    148  YES                                                                              2.65   3.55   11    15    3.15                                                                             off                                              2.80   3.95   12    30       scale                                    __________________________________________________________________________

As the data demonstrates, heat setting has no roughening effect on thefilm, and may even be responsible for reducing the surface roughnesssomewhat.

In light of Examples 135-148, it appears that the unexpected surfaceroughness observed on the film line films, containing none of theparticulate slip agents customarily used in biaxially oriented polyesterfilms, is not due to the film casting process, the simultaneous biaxialstretching process (even when preceded by a pre-stretching in themachine direction), or the heat setting process.

EXAMPLE 149-191

The following examples illustrate the effect of tenter preheat on hazeand roughness.

Additional experiments were performed at conditions of Examples 126-134,to determine which, if any, of the process variables had significanteffects on the surface roughness of the film, as characterized by hazemeasurement. The process variables investigated were the temperature ofthe heated rolls in the length orienter (T_(LO)), the stretch ratio inthe length orienter (SR_(LO)), the temperature in the preheat zone ofthe tenter (T_(PH)), the temperature in the stretch zone of the tenter(T_(STR)), the temperature in the first heat-set zone of the tenter(T_(STR)), the temperature of the second heat-set zone of the tenter(T_(HS2)), the transverse direction stretch ratio in the stretch zone ofthe tenter as measured by grip separation (SR_(TD)), and the transversedirection stretch ratio after relaxation, as measured by the gripseparation at the tenter exit (SR_(REL)).

In the length orienter, the idler rolls were maintained consistently at6° C. warmer than the slow driven rolls. Thus, only the temperature ofthe driven rolls is listed in Table 14. In some examples, the lengthorienter was bypassed altogether to examine the effect of using only thesimultaneous-biaxial tenter to stretch the film.

Table 14 contains the experimental conditions, the measured values forHaze, and some measured values for surface roughness. The latter wereobtained by the Rodenstock method, and represent the average value ofboth sides. The table is arranged in order of increasing preheat zonetemperature, and some of Examples 126-134 are relisted for clarity.

                                      TABLE 14                                    __________________________________________________________________________                                       Rod'c                                      Ex. T.sub.LO                                                                             T.sub.PH                                                                         T.sub.STR                                                                         T.sub.HS1                                                                        T.sub.HS2  Haze                                                                             Ra                                         No. (°C.)                                                                     SR.sub.LO                                                                         (°C.)                                                                     (°C.)                                                                      (°C.)                                                                     (°C.)                                                                      SR.sub.TD                                                                        SR.sub.REL                                                                        (%)                                                                              (nm)                                       __________________________________________________________________________    149 none                                                                             none                                                                              153                                                                              153 193                                                                              216 4.38                                                                             4.02                                                                              1.1                                           150 102                                                                              131 154                                                                              156 182                                                                              216 4.40                                                                             4.03                                                                              1.1                                           151 none                                                                             none                                                                              157                                                                              156 193                                                                              216 4.38                                                                             4.02                                                                              1.8                                           152 none                                                                             none                                                                              157                                                                              156 193                                                                              216 4.62                                                                             4.24                                                                              1.8                                           153 102                                                                              1.31                                                                              159                                                                              156 177                                                                              216 4.40                                                                             4.03                                                                              2.6                                           154 118                                                                              1.41                                                                              160                                                                              160 216                                                                              199 4.98                                                                             4.98                                                                              4.6                                           155 102                                                                              1.31                                                                              161                                                                              156 177                                                                              216 4.40                                                                             4.03                                                                              3.7                                           156 none                                                                             none                                                                              161                                                                              156 193                                                                              216 4.62                                                                             4.24                                                                              4.2                                                                               27                                        157 118                                                                              1.35                                                                              161                                                                              160 204                                                                              193 4.48                                                                             4.48                                                                              5.3                                           158 118                                                                              1.34                                                                              161                                                                              160 215                                                                              198 4.98                                                                             4.98                                                                              5.8                                           159 118                                                                              1.44                                                                              161                                                                              160 215                                                                              198 4.48                                                                             4.48                                                                              6.3                                           160 124                                                                              1.41                                                                              161                                                                              160 215                                                                              198 4.98                                                                             4.98                                                                              8.4                                           161 118                                                                              1.34                                                                              161                                                                              160 215                                                                              198 4.48                                                                             4.48                                                                              13.5                                          162 118                                                                              1.34                                                                              161                                                                              160 215                                                                              198 4.73                                                                             4.73                                                                              15.1                                          163 118                                                                              1.35                                                                              161                                                                              160 204                                                                              193 4.48                                                                             4.23                                                                              15.3                                          164 118                                                                              1.33                                                                              162                                                                              149 232                                                                              199 5.31                                                                             5.10                                                                              8.6                                           165 118                                                                              1.33                                                                              162                                                                              149 232                                                                              199 5.08                                                                             4.91                                                                              11.0                                          166 118                                                                              1.33                                                                              162                                                                              149 232                                                                              199 5.08                                                                             4.88                                                                              15.7                                          167 118                                                                              1.35                                                                              163                                                                              163 193                                                                              216 4.62                                                                             4.49                                                                              6.7                                                                               71                                        168 118                                                                              1.35                                                                              163                                                                              163 193                                                                              193 4.62                                                                             4.49                                                                              7.7                                           169 118                                                                              1.35                                                                              163                                                                              163 216                                                                              193 4.62                                                                             4.49                                                                              8.3                                                                               84                                        170 118                                                                              1.35                                                                              163                                                                              163 216                                                                              216 4.62                                                                             4.24                                                                              8.8                                                                              126                                        171 118                                                                              1.35                                                                              163                                                                              163 216                                                                              193 4.62                                                                             4.24                                                                              8.9                                                                               83                                        172 118                                                                              1.35                                                                              163                                                                              163 193                                                                              216 4.62                                                                             4.24                                                                              9.1                                                                              102                                        173 118                                                                              1.33                                                                              163                                                                              149 215                                                                              198 5.08                                                                             4.95                                                                              9.9                                           174 118                                                                              1.35                                                                              163                                                                              163 204                                                                              193 4.62                                                                             4.36                                                                              10.2                                                                             113                                        175 118                                                                              1.35                                                                              163                                                                              163 204                                                                              204 4.62                                                                             4.36                                                                              10.3                                                                             114                                        176 118                                                                              1.31                                                                              163                                                                              163 216                                                                              199 4.98                                                                             4.98                                                                              11.8                                          177 118                                                                              1.33                                                                              163                                                                              149 215                                                                              198 4.54                                                                             4.42                                                                              12.3                                          178 118                                                                              1.35                                                                              163                                                                              163 216                                                                              216 4.62                                                                             4.49                                                                              12.7                                                                             208                                        179 118                                                                              1.35                                                                              163                                                                              163 204                                                                              204 4.62                                                                             4.24                                                                              14.8                                                                             118                                        180 118                                                                              1.41                                                                              163                                                                              163 216                                                                              199 4.98                                                                             4.98                                                                              17.0                                          181 118                                                                              1.28                                                                              163                                                                              163 216                                                                              199 4.54                                                                             4.41                                                                              26.6                                          182 118                                                                              1.35                                                                              163                                                                              156 177                                                                              216 4.40                                                                             4.03                                                                              5.7                                           183 116                                                                              1.35                                                                              163                                                                              156 177                                                                              216 4.40                                                                             4.03                                                                              6.9                                           184 110                                                                              1.31                                                                              163                                                                              156 177                                                                              216 4.40                                                                             4.03                                                                              8.1                                           185 113                                                                              1.35                                                                              163                                                                              156 177                                                                              216 4.40                                                                             4.03                                                                              8.6                                           186 107                                                                              1.31                                                                              163                                                                              156 177                                                                              216 4.40                                                                             4.03                                                                              8.7                                           187 102                                                                              1.31                                                                              163                                                                              156 177                                                                              216 4.40                                                                             4.03                                                                              9.8                                           188 none                                                                             none                                                                              163                                                                              156 193                                                                              216 4.62                                                                             4.24                                                                              12.4                                          189 none                                                                             none                                                                              166                                                                              157 193                                                                              216 4.62                                                                             4.24                                                                              4.2                                           190 none                                                                             none                                                                              166                                                                              160 193                                                                              216 4.62                                                                             4.24                                                                              12.3                                          191 124                                                                              1.34                                                                              168                                                                              148 213                                                                              199 5.02                                                                             4.99                                                                              28.7                                          __________________________________________________________________________

Standard statistical analysis of this data indicates that the mostsignificant process variable with respect to haze is the temperature inthe preheat zone of the tenter. This is made clearer in Table 15 below,which shows the average value of haze for each value of T_(PH),regardless of the values of the other process parameters.

                  TABLE 15                                                        ______________________________________                                                T.sub.PH                                                                           Haze                                                                     (°C.)                                                                       (%)                                                              ______________________________________                                                153  1.1                                                                      154  1.1                                                                      157  1.8                                                                      159  2.6                                                                      160  4.6                                                                      161  8.6                                                                      162  11.7                                                                     163  10.7                                                                     166  8.3                                                                      168  28.7                                                             ______________________________________                                    

An effect on haze of secondary importance is observed in the data ofExamples 182-188. From these examples, it can be seen that raising thetemperature of the heated rolls in the length orienter serves to reducethe haze in the case of tenter preheat zone and stretch zonetemperatures of 163° and 156° C., respectively.

Without wishing to be bound by any particular theory, it appears thatthe surface roughness and haze of PEN:PET multilayer films containingPEN as each surface layer, is caused by the crystallization of PETlayers during preheating (before stretching), and subsequent breakup andrearrangement of the PET crystallites during stretching. In the absenceof any stretching in a length orienter prior to the simultaneous-biaxialtenter, the PET layers crystallize to a greater extent as the preheattemperature is raised. The thus-formed crystallites in the PET layersnearest to the surface are separated from one another during the biaxialstretching step, and serve to provide surface roughness through theoutermost PEN layer, much as marbles might provide visible lumps ifplaced under a carpet. If the film is first stretched somewhat in alength orienter, the increased temperature in the length orienter mayserve either to inhibit the formation of large PET crystallites in thetenter preheat zone, or to promote the deformation upon subsequentbiaxial stretching of those which do form.

EXAMPLES 192-201

The following examples illustrate the effect of preheating time onsurface roughness, haze, film color and modulus.

The single aspect of a film line most difficult to simulate in alaboratory stretching apparatus is the time-temperature history of thefilm as it traverses the film line. This difficulty is inherent in thedifference between moving a web from chamber to chamber, each maintainedat a different temperature (film line), and changing the temperature ofthe surrounding air in a single chamber (laboratory film stretcher).This time-temperature history, particularly the preheating time prior tothe simultaneous biaxial stretching step, is a significant differencebetween the film line conditions and the laboratory simulations.

A series of experiments was therefore performed to explore the effect ofvarying the preheating time prior to stretching. Specimens of the castweb retained from the film line experiment (Experiment 122) wereprepared for laboratory stretching. All were stretched in bothdirections simultaneously at 100%/sec to a biaxial stretch ratio of 5.5at 150° C. The amount of time allowed for preheating the undrawnspecimen at 150° C. was varied in 5 second increments from 0 to 45seconds (45 seconds was the value used in all of the precedinglaboratory stretching Examples). In addition, for each preheat durationexamined, a second cast web sample was mounted in the laboratorystretcher, preheated, and removed immediately without undergoing thesimultaneous biaxial stretch.

The preheated but unstretched specimens were examined visually, side byside, for haziness. At 150° C., the PET layers would be expected tocrystallize into a spherulitic morphology, causing haze or whitening.This process would be expected to be much slower for theslower-crystallizing PEN layers. Thus, an increase in haze in thepreheated but unstretched web specimens can be attributed tocrystallization of the PET layers. Several specimens were examined "onedge" under a microscope, and it was confirmed that the haziness orwhitening occurred only in the PET layers. The stretched films were alsoinspected visually, side by side, for haziness. Those experienced in theart recognize that haze in finished film can be highly correlated tosurface roughness, especially at the high levels of surface roughnessexhibited in Examples 135-137. The data of Table 14 serves tocorroborate this relationship. Thus, the qualitative assessment of hazein the stretched films was taken as an indication of surface roughness.The films were also inspected visually for color/iridescence. Thepresence of bands of color running along the specimen's original machinedirection, or, alternatively, uniform iridescence, was noted.

Modulus measurements were taken in both the machine and transversedirections. Since the films had been equally and simultaneouslybiaxially drawn, these modulus results were averaged over the twodirections. The results are shown in TABLE 16.

                  TABLE 16                                                        ______________________________________                                        Exam- Preheat           Stretched      Modulus,                               ple   time,   Unstretched                                                                             Film   Stretched                                                                             kpsi                                   No.   sec.    Haze      Haze   Film Color                                                                            (10.sup.6  kPa)                        ______________________________________                                        192    0      None      --     --      --                                     193    5      None      None   Banded   976 (6.73)                            194   10      None      None   Banded   977 (6.74)                            195   15      Slight    Some   Banded   982 (6.77)                            196   20      Increased Maximum                                                                              Less Banded                                                                           1064 (7.34)                            197   25      Increased Some   Less Banded                                                                           1060 (7.31)                            198   30      Increased Some   Less Banded                                                                           1051 (7.24)                            199   35      Increased None   Iridescent                                                                            1042 (7.18)                            200   40      Increased None   Iridescent                                                                            1051 (7.25)                            201   45      Unchanged None   Iridescent                                                                            1020 (7.03)                            ______________________________________                                    

Examining these results, it is clear that the PET layers crystallizeincreasingly with preheating time, perhaps leveling off at 40-45seconds. However, stretched film haze and, by extension, surfaceroughness, goes through a maximum at about 20 seconds preheat time,eventually disappearing for the specimens preheated for about 35 secondsor more. The disappearance of haze is accompanied by the dissolution ofthe color banding into uniform overall iridescence. Recalling that thefilm line tenter conditions of Example 122 provided a preheating time ofabout 18 seconds, and only 6 seconds more in the stretch zone, itappears likely that this is the cause of the color banding and hazenoted in Examples 122-124, and thus, the surface roughness observed inExamples 135-137.

Examination of the data in Table 16 also leads to the conclusion thatthere are at least two accessible "levels" of stretched film modulus,depending on duration of preheating. The films from Examples 193-195(5-15 second preheat time) had a modulus of about 980 kpsi (6.76×10⁶kPa) The films from Examples 196-200 (20-40 second preheat time) had amodulus of about 1050 kpsi (7.24×10⁶ kPa). This suggests that themodulus may be beginning to decline at still longer preheat durations.

Without wishing to be bound by any particular theory, the followingexplanation for these observations appears plausible: The PET layers inthe multilayer cast web begin to crystallize during the preheating stepin the simultaneous biaxial tenter or lab stretcher. If the film isstretched before this process has had enough time to result in asignificant number of spherulitic structures of sizes larger thanoptical wavelengths, such structures do not form during the stretchingstep either, and the resulting film remains clear. Because the preheatedbut unstretched web consists of largely amorphous layers of both PEN andPET, and because the stretching temperature is so much higher than theTg of PET, the PET layers deform without significant strain-hardening(i.e., there is viscous flow), and contribute relatively less to theoverall modulus of the stretched film.

If, however, the PET layers are allowed to spherulitically crystallizeto a moderate extent before stretching commences, a sufficiententanglement network, anchored by crystallites, exists in the PET toeffectively transmit stretching forces and cause strain-hardening in thePET layers. This results in a relatively increased contribution of thePET layers to the overall modulus of the stretched film, but doesnothing to disrupt the spherulitic structures already formed. Thus, thepreheated web's haze remains in the stretched film. Ultimately, if thePET layers are allowed to crystallize still further, thecrystallite-anchored entanglement network is strong enough to transmitstretching forces and cause strain-hardening, and to disrupt thepre-existing spherulitic structures in the PET layers. The efficiency ofthe network in transmitting stretching forces is indicated by thedissolution of the color banding into uniform iridescence, which impliesthat local thickness and/or orientational gradients have disappeared.The disruption of the spherulites is implied by the disappearance ofhaze during the stretching step. For haze to disappear, structures largeenough to diffract light must be broken up or otherwise reformed intostructures of much smaller size. This is observed in the uniaxial and/orbiaxial orientation of some semicrystalline polymers such aspolyethylene and polypropylene, both of which can be stretched while inthe semicrystalline state, and can be made to clarify to some extent dueto the reorganization of spherulites and large lamellar bundles intosmaller lamellar bundles or fibrillar or rodlike structures.

PET, however, is known not to be highly stretchable once crystallizedinto spherulitic structures, and has not previously been observed toclarify during orientational stretching. This unexpected result,combined with the observation, in the discussion accompanying Examples45-57, of the consistency of the observed modulus values with anunprecedented level of modulus within the PET layers, argues that theorientation of the PET layers in the PEN:PET multilayer compositionsoccurs by a unique and novel mechanism for orientational deformation ofPET.

Additional insight into the utility of the multilayer construction forpromoting this deformational mechanism can be gained by furtherexamination of the differences between PEN-surfaced and PET-surfacedmultilayer films. In Examples 114-117 and 138-140, it was observed thatthe PET-surfaced films were rougher, slipperier, and hazier thanPEN-surfaced films of similar composition. This can be interpreted as amanifestation of the uniqueness of the PET surface layers, compared tointernal PET layers in a multilayer construction. Having no overlyingPEN layer on one surface, outermost PET layers behave more likeconventional free-standing PET films. After crystallizing in a preheatstep, stretching causes them to break up, resulting in a patchy, frostyhazed appearance, high (often off-scale) surface roughness, and very lowcoefficients of friction.

On the other hand, PET layers in the interior of the multilayerconstruction stretch, without breaking, to stretch ratios much higherthan those commonly observed for the biaxial orientation offree-standing monolayer PET films. Depending on the preheatingconditions, spherulites may or may not break up or deform into smallerstructural units. If not, they provide a "lumpiness" underneath the PENsurface layer, which results in surface roughness in much the same waythat placing marbles under a carpet would create a bumpy floor covering.

It will be clear to one skilled in the art, from the foregoingdiscussion, that the level of surface roughness will be controllable by,among other things, the time-temperature history of the cast web priorto the beginning of stretching, and the details of construction of themultilayer film. The latter include, but are not limited to, theproportion of the two polymers in the construction, the thickness of thePEN surface layers, and the thicknesses of the PET layers nearest thesurface. As such, the constructions of the present invention alsoconstitute, unexpectedly, a unique and novel "slip" system for polyesterfilms, which is not dependent on the addition of any particulatesubstances in any amount.

EXAMPLES 202-203

The following examples corroborate the assumption of an efficiententanglement network with crystalline junctions in the well-crystallizedPET layers obtained through long preheat times.

The laboratory stretcher was equipped with force transducers on abouthalf of the grippers, so that stretching force data could be obtained.The stretcher was also adjusted so that a nominal stretch ratio of 6.25(rather than 6.0) could be achieved. Specimens for stretching wereprepared from the retained cast web of Example 122. Stretching was onceagain done in the simultaneous biaxial mode, at 100%/sec in eachdirection, to a biaxial draw ratio of 6.25 at 150° C., after preheatingat the same temperature.

Example 202 was stretched after preheating for 45 seconds, and Example203 was stretched after preheating for only 10 seconds. At theseconditions, both cast web specimens should be thoroughly preheatedthroughout their thickness, but the specimen of Example 202 should havewell-crystallized PET layers, while the specimen of Example 203 shouldhave almost no crystallinity. Since the stretching experiments wereperformed equally and simultaneously in both the machine and transversedirections, the output from all force transducers was averaged for eachexample.

The results of the stretching experiments are shown in FIG. 15. It isreadily apparent that there are two main differences between thestress-strain traces. First, Example 202 exhibits a sharp sudden rise inforce immediately upon the commencement of draw, which is not present inExample 203. Secondly, once strain-hardening commences at a draw ratioof about 3.0, the slope of Example 202 rises faster than that of Example203.

These results are consistent with the interpretation that thecrystalline structures in the PET layers of the specimen in Example 202must initially be broken up, requiring considerable force. Theuncrystallized PET layers in the specimen of Example 203 require no suchhigh force to deform. Further, the steeper rise in the strain-hardeningregion in Example 202 is consistent with an interpretation of moreefficient orientational deformation resulting in strain-hardening of thePET layers as well as the PEN layers.

This interpretation leads to the conclusion that the uncrystallized PETlayers of the specimen of Example 203 contribute negligibly to theoverall stretching stress. This implication can be tested by resealingthe stress trace of Example 203. Since the specimen is 80% PEN and 20%PET, if the PET contributes negligibly, the entire specimen would beexpected to behave similarly to a monolayer specimen of PEN having 80%of the cast thickness. Since stress is force divided by cross-sectionalarea, this is equivalent to rescaling the stress upwards by 125%. Thisis shown in FIG. 16, in which the stress trace for Example 203 has beenboth rescaled and shifted upward for clarity to match the trace ofExample 202 in the plateau region.

These results confirm that the PET layers, if not crystallized, largelydeform during stretch by non-strain hardening means (viscous flow). Whencrystallized through sufficient preheating, however, the PET layersdeform first by destruction or re-organization of the existing crystalstructure, followed by strain-hardening similar to that occurring in thePEN layers.

EXAMPLES 204-228

The following examples illustrate the effect of preheating conditionsduring length orientation on haze and uniformity.

Since the design of the film line being used for these studies required,in order to obtain sufficient machine direction stretch ratios, a lengthorientation step prior to the simultaneous biaxial tenter, it was ofinterest to explore the effects of preheating conditions on the lengthorienter step as well. The patent literature regarding sequentiallybiaxially oriented PEN films indicates that the preferred temperaturesfor the machine direction stretching step is not as high as 150° C., theoptimum temperature for simultaneous biaxial draw of the multilayerfilms as indicated by laboratory results. Therefore, both the preheatingtemperature and time were studied.

In Examples 204-228, specimens of the retained cast web of Example 122were mounted in the laboratory stretcher in such a way as to be grippedonly in the machine direction. The other two sides remained ungripped,and were thus free to contract as they are in a length orienter. Foreach specimen, the preheat and machine direction stretch temperaturewere the same. Temperature was varied over the range 120°-170° C., andthe preheat times employed were 7 seconds (the best estimate of the timerequired for the surfaces of the specimen to reach the preheat/stretchtemperature), 15 seconds (as an estimate of the time required for thespecimen to approach the preheat/stretch temperature throughout itsthickness), and 45 seconds (the standard preheat time used in most priorlab stretcher experiments. The conditions tested are shown in Table 17,which shows the example number for each set of variables explored.

                                      TABLE 17                                    __________________________________________________________________________    Preheat                                                                           Preheat/Stretch Temp. (C.)                                                Time                                                                              120 125                                                                              130 135                                                                              140 145                                                                              150 155                                                                              160 170                                       (sec)                                                                             Ex. No.                                                                   __________________________________________________________________________     7  204 205                                                                              206 207                                                                              208 209                                                                              210 211                                                                              212 --                                        15  213 214                                                                              215 216                                                                              217 218                                                                              219 -- --  --                                        45  220 221                                                                              222 223                                                                              224 225                                                                              226 -- 227 228                                       __________________________________________________________________________

Machine direction stretching was done at 100%/sec to a stretch ratio of1.50. Ink marks were made on each specimen, so that the uniformity ofdeformation of each could be judged. After all the specimens had beenstretched, they were assessed visually for stretch uniformity andwhitening (haziness). For each set created with the same preheat time,it was observed that there was some central value(s) or preheat/stretchtemperature at which the stretching uniformity was best, and stretchinguniformity degraded continuously as temperature was raised or lowered.For haze, it was observed in each set that there was a preheat/stretchtemperature at which haze first appeared, and raising the temperaturecaused a continuous increase in the haze until the specimens becamequite white. The results are summarized in Table 18.

                                      TABLE 18                                    __________________________________________________________________________    Preheat                                                                       Time                                                                              Preheat/Stretch Temp. (C.)                                                (sec)                                                                             120                                                                              125                                                                              130  135  140   145   150                                                                              155                                                                              160  170                                __________________________________________________________________________     7                  Best  Best        Onset of                                                    Stretch                                                                             Stretch     Haze                                                        Uniformity                                                                          Uniformiy                                           15                  Best  Onset of                                                                Stretch                                                                             Haze                                                                    Uniformity                                                45        Onset of                                                                           Best Best                                                                Haze Stretch                                                                            Stretch                                                                  Unifority                                                                          Uniformity                                                __________________________________________________________________________

One can clearly see from these results that the temperature for beststretch uniformity, an important consideration in a length orienter, isinversely related to preheat time. Thus, as the preheat time isincreased, the temperature for best stretch uniformity slowly falls from140°-145° C. to 140° C. to 135°-140° C. The onset of haze, however, is astrong function of the preheat time, eventually occurring attemperatures lower than the optimum temperatures for uniform stretching.It is clear, however, that at sufficiently short preheat times, auniform length orientation stretch can be performed without the onset ofhaze. In fact, no haze was observed in the film between the lengthorientation and tenter in the experiments of Examples 122-134, 143-146,or 149-191.

EXAMPLE 229

The following examples illustrate the crystallizability of PET in alength oriented web.

The film of Example 208, preheated for 7 seconds at 140° C. prior tomachine direction stretch to stretch ratio of 1.5, was further heatedwhile gripped in the machine direction for 45 seconds at 150° C. The PETlayers of the clear machine direction-stretched film whitened similarlyto the cast web sample of Example 201. This confirms the feasibility ofproducing conditions in the tenter-preheated web conducive to makingclear, smooth, high modulus films even when the tentering step ispreceded by a length orientation step.

EXAMPLES 230-235

The following examples illustrate the properties of cast webs made withdifferent numbers of layers.

Additional cast web rolls were made by techniques similar to those ofExamples 1-24 and 89-103 using 13/4 inch extruders for both PEN and PET.The PEN resin IV was about 0.50, and the PET resin IV was about 0.80.Short, 3/4 inch neck tubes were used to transport the extrudates to themultilayer feedblock. A 12-inch wide Cloeren film die was used.Different modular inserts were used in the feedblock in the variousExamples, each designed to provide a multilayer film of an odd number ofalternating layers: 3, 7, 13, 29, and 61. The feedblock inserts were notmodified to provide doubly-thick outer layers as had been done inseveral previous examples. All cast webs were made with PEN as theoutermost layers.

The PEN resin was dried at about 177° C. and extruded at about 293° C.The PET resin was dried at about 138° C. and extruded at about 282° C.The neck tubes were maintained at about 293° C. and 277° C.,respectively. The feedblock and die were maintained at about 282° C. Thecasting roll was maintained at about room temperature. Total throughputwas about 80 lbs./hr., and each composition was about 80% PEN and castto about 15 mils. The exact figures are given in Table 19.

Of the cast webs made with each feedblock insert, those having the bestappearance were rolled and retained for later experimentation. The bestcast web made in these experiments with the 13 and 61-layer inserts hadrheologically-related surface defects. In order to make validcomparisons, some webs made with the 29 layer insert were rolled up andretained even though they, too, had some surface defects. A roll madewith the 29 layer feedblock without defects was also obtained. Detailsare given in Table 19.

                  TABLE 19                                                        ______________________________________                                                 Number of        Cast Thickness                                      Example No.                                                                            Layers   % PEN   (mils)   Quality                                    ______________________________________                                        230       3       80      15.8     Good                                       231       7       81      153      Good                                       232      13       81      15.1     Slight Defects                             233      29       81      18.0     Good                                       234      29       82      16.3     Defects                                    235      61       80      15.2     Defects                                    ______________________________________                                    

EXAMPLES 236-243

The following examples illustrate the effect of the number of layers onstretchability.

Specimens were prepared for laboratory stretching from the cast webs ofExamples 230-235. In addition, specimens were prepared from twodifferent cast webs of monolayer PEN to serve as "controls". One was thecast web of Example 1. This web had a similar thickness to those ofExamples 230-235, but used PEN of a higher IV. A second control web wasmonolayer PEN retained from the start-up of the experiment of Examples126-134, extruded at the conditions cited for PEN therein. This web wasthinner (9.7 mils), but matched the PEN IV of Examples 230-235.

The laboratory film stretcher was used with the added force transducerinstrumentation to determine UBSRs. Stretching was done as usual at 150°C., after 45 seconds preheating, at 100%/sec in both the machinedirection and the transverse direction simultaneously. The specimenswere all stretched to a nominal biaxial stretch ratio of 6.25. If aspecimen broke before stretching that far, the stress-strain trace forthe experiment showed a sudden fall at the instant of specimen failure.The resolution of the instrument was about 0.12 stretch ratio units, andthe precision was about 0.02 units.

For each material, five specimens were stretched. The highest value forstretch ratio replicated within the five tests is considered to be theUBSR. If no value was repeated in five tests, additional tests wereperformed until a value in the upper half of all values was replicated.This procedure eliminates contamination of the data by extraneouseffects (i.e., nicks in the specimen edges). In most cases, replicationis achieved the highest or second-highest value obtained. The resultsare shown in TABLE 20.

                  TABLE 20                                                        ______________________________________                                        Example                                                                       No.    Cast Web No.                                                                             No. of Layers                                                                             Comments UBSR                                   ______________________________________                                        236     1         Monolayer PEN                                                                             Higher IV                                                                              5.51                                   237    237        Monolayer PEN                                                                             Thinner Caliper                                                                        5.40                                   238    230         3          --       5.63                                   239    231         7          --       6.00                                   240    232        13          Slight Defects                                                                         6.24                                   241    233        29          --       6.23                                   242    234        29          Defects  6.11                                   243    235        61          Defects  6.24                                   ______________________________________                                    

Results of 6.23 or 6.24 were obtained from fully successful 6.25×stretches, the difference reflecting only the precision of theinstrument. It is clear from the data presented in Table 20 that theresults at 13, 29, and 61 layers are roughly equivalent, given theconstraints of the laboratory stretcher. It could be argued that theresults at 61 layers are superior to those at 29, since surface defectsdid not degrade performance to a level below the stretching machinelimitation. However, the results at 7 layers are significantly lessimpressive, and those at 3 approach those of plain monolayer PEN films.

These results imply that the enhanced stretchability effect inmultilayer films of the present invention is improved by increasing thenumber of layers at least to 13, and perhaps beyond. A significanteffect is still seen at layer numbers as low as 7, but the effect on 3layer films is negligible.

EXAMPLE 244-249

The following examples illustrate USBRs obtained for 13-layer films.

Additional cast web rolls were made, and specimens from them stretched,by techniques similar to those in Examples 230-243. Only the 13 layerfeedblock insert was used. Cast webs were made at about 60, 70, 75, 80,85, and 90% PEN. Cast caliper was controlled at about 10 mils, so as tobe comparable to the monolayer PEN of Example 237. Stretching andassessment of UBSR was done as in Examples 236-243. The details andresults are shown in Table 21, with Example 237 repeated for clarity.

                  TABLE 21                                                        ______________________________________                                        Example            Cast Thickness                                                                           Cast Web                                        No.    % PEN       (mils)     Surface Defects                                                                        UBSR                                   ______________________________________                                        244    61          10.3       Moderate 5.76                                   245    70          10.3       Moderate 6.00                                   246    75          10.5       Moderate 6.12                                   247    81          10.0       Slight   6.24                                   248    84          10.2       Slight   6.00                                   249    91           9.9       Moderate 5.76                                   237    Monolayer PEN                                                                              9.7       None     5.40                                   ______________________________________                                    

It is clear from the table that the 13 layer films exhibit the sametrend found in the 29 layer series of Table 3 and FIG. 3. The absolutevalues of the UBSRs differ because of the different measurementtechniques employed. Still, the enhanced stretchability clearly goesthrough a maximum for both data sets at about 80% PEN, and stretchingperformance is as good or better than for monolayer PEN at allcompositions greater than about 60% PEN.

EXAMPLE 250-251

The following examples illustrate the production of tensiled multilayerfilms.

An effort was made to make "tensilized" films (films with a machinedirection modulus significantly higher than a transverse directionmodulus) on the film line. Conditions were similar to those of Example122 , with the following exceptions. PET was dried about 129° C. The PETmelt train was maintained at about 271° C. One inch (2,54 cm) neck tubeswere used. The 12 inch (30.5 cm) wide Cloeren film die of Examples230-235 was used. The feedblock was maintained at the same temperatureas the die (about 288° C.). The casting roll was maintained at about 32°C. The webs were cast at thicknesses of 13 and 9 mils, respectively, forExamples 250 and 251. All the heated rollers of the length orienter weremaintained at the same temperature, about 107° C. The stretch ratio inthe length orienter was limited to 1.04. The preheat and stretch zonesin the tenter were maintained at about 155° C. and 149° C.,respectively. The nominal stretch ratios in the stretch zone of thetenter were 4.40 and 4.53 in the machine direction and transversedirection, respectively.

The tenter was equipped with a modification permitting, immediatelyfollowing the simultaneous biaxial stretch, a secondary stretch in themachine direction at a stretch ratio of 1.09. Thus, the total stretchratio in the machine direction was 1.04×4.40×1.09, or 4.99. Real drawratios measured via the displacement of ink marks on the webs were 5.15and 5.10 in the machine and transverse directions, respectively. Thefirst heat-set zone was maintained at about 210° C., and the secondheat-set zone was maintained at about 204° C. The cooling zone wasmaintained at about 66° C.

The film was relaxed under restraint similarly to Examples 126-134,except that all the relaxation occurred in the cooling zone. The relaxednominal transverse direction stretch ratio was 4.24.

The thickness, Green Modulus, heat shrinkage, haze, and surfaceroughness (by Rodenstock) of the films is shown in Table 22. Roughnessvalues are given for both sides of each film. In appearance, both of thefilms were slightly hazy.

                                      TABLE 22                                    __________________________________________________________________________                         150° C./                                                                    150° C./                                              Green Mod.                                                                          Green Mod.                                                                          15 min                                                                             15 min                                                       MD,   TD,   Shrinkage                                                                          Shrinkage                                                                             Rodenstock                                                                          Rodenstock                            Example                                                                            Caliper                                                                           kpsi  kpsi  MD   TD   Haze                                                                             Ra    Rq                                    No.  (mils)                                                                            (10.sup.6 kPa)                                                                      (10.sup.6 kPa)                                                                      (%)  (%)  (%)                                                                              (nm)  (nm)                                  __________________________________________________________________________    250  0.47                                                                              1036  733   3.76 -(0.12)                                                                            7.13                                                                             144   210                                            (7.14)                                                                              (5.05)             170   240                                   251  0.32                                                                               996  721             6.26                                                                              72   104                                            (6.87)                                                                              (4.97)              92   132                                   __________________________________________________________________________

The data shows that the "secondary stretching" modification to the linefilm line was successful in producing tensilized film. Compared to theresults of Examples 126-134 in Table 10, the machine direction GreenModuli are about 250-300 kpsi (1.02-2.07×10⁶ kPa) higher, the transversedirection moduli are roughly unchanged, the MD shrinkage is, asexpected, somewhat higher, and the TD shrinkage remains near zero. Hazeis roughly equivalent to the best Examples in Table 10. These resultsindicate that multilayer tensilized films can be made by the techniqueof these examples.

EXAMPLES 252-259

The following examples illustrate that the multilayer effect of enhancedstretchability applies to both sequential drawing processes as well asto simultaneous drawing processes.

Cast webs from Examples 122 (25 Layer, 80% PEN multilayer) and Example237 (Monolayer PEN) were used to explore the question of whether theenhanced stretchability of the multilayer films also applies to the moreindustrially common sequential stretching process. Conditions forstretching were as before: 45 second preheat at the stretchingtemperature, 100%/sec stretch rate in each direction. The specimens werestretched sequentially, first in the original machine direction of thecast web, then in the transverse direction, without any pause betweenstretching steps.

The monolayer PEN of Example 237 was examined first to determine itsstretching behavior in the sequential mode. The preheat/stretchtemperature was varied in 5° C. increments from 120°-150° C. At eachtemperature, the lab stretcher was set to stretch to the same specificstretch ratio in both directions sequentially. If the specimen broke,the experiment was repeated with lower stretch ratios. If the specimendid not break, the experiment was repeated with higher stretch ratios.The stretch ratio increment was 0.1 stretch ratio units.

When the borderline between successful and unsuccessful stretches wasestablished and reproduced, the highest successful value of stretchratio was deemed the sequential-mode UBSR. The films were also evaluatedfor stretch uniformity. Those deemed non-uniform typically stretchednon-uniformly in the second or transverse direction, leaving thick andthin bands than ran along the machine direction. The exception wasExample 252, which stretched non-uniformly in the first, or machinedirection, step. The results are given in Table 23.

                  TABLE 23                                                        ______________________________________                                        Example No.                                                                            Stretch Temp. (°C.)                                                                 UBSR    Comment                                         ______________________________________                                        252      120          4.0     Non-Uniform in MD                               253      125          4.3     Good                                            254      130          4.6     Good                                            255      135          4.4     Non-Uniform in TD                               256      140          4.0     Non-Uniform in TD                               257      145          4.1     Non-Uniform in TD                               258      150          4.4     Non-Uniform in TD                               ______________________________________                                    

These results show that the optimum temperature for stretchability forPEN is about 130° C. This is consistent with existing prior art. At 130°C., the sequential-mode UBSR is highest and the film is uniform. UBSRfalls off in each direction from 130° C., but rises again at 145°-150°C., as the effects of stretching an uncrystallized but overheated webbegin to result in a "melty" stretch.

The mutilayer sample was then stretched at the optimum PEN temperatureof 130° C. using the same protocols. This is Example 259. Thesequential-mode UBSR for the cast web of Example 122 was found to be inexcess of 5.0. Thus, the multilayer effect of enhanced stretchabilitydoes apply to the sequential drawing process as to the simultaneousprocess.

The preceding description is meant to convey an understanding of thepresent invention to one skilled in the art, and is not intended to belimiting. Modifications within the scope of the invention will bereadily apparent to those skilled in the art. Therefore, the scope ofthe invention should be construed solely by reference to the appendedclaims.

What is claimed is:
 1. A method for making a multilayer polyester film,comprising the steps of:providing a multilayer film comprising first,second, and third layers, wherein at least one of the first and thirdlayers comprises a naphthalene dicarboxylic acid polyester and whereinthe second layer comprises a terephthalic acid polyester; and orientingthe film in at least one direction to a stretch ratio R⁶ such thatR⁶ >k, where k is the maximum stretch ratio obtainable, under the samestretch conditions, for a monolithic film of the naphthalenedicarboxylic acid polyester having the same dimensions.
 2. The method ofclaim 1, wherein the multilayer film contains at least 7 layers whichcomprise a resin selected from the group consisting of naphthalenedicarboxylic acid polyesters and terephthalic acid polyesters.
 3. Themethod of claim 1, wherein the multilayer film contains at least 13layers which comprise a resin selected from the group consisting ofnaphthalene dicarboxylic acid polyesters and terephthalic acidpolyesters.
 4. The method of claim 2, wherein each layer of the filmcomprises a resin selected from the group consisting of naphthalenedicarboxylic acid polyesters and terephthalic acid polyesters.
 5. Themethod of claim 1, further comprising the step of:stretching at leastone layer comprising the first resin to a stretch ratio of at leastabout 5.5.
 6. The method of claim 2, wherein each layer of the filmcomprises a resin selected from the group consisting of naphthalenedicarboxylic acid polyesters and terephthalic acid polyesters.
 7. Themethod of claim 1, wherein the film is oriented in at least twodirections.
 8. The method of claim 7, wherein the orientation isperformed sequentially in the at least two directions.
 9. The method ofclaim 7, wherein the orientational stretch ratios in both of the atleast two directions are at least about 4.6.
 10. The method of claim 7,wherein the orientational stretch ratios in both of the at least twodirections are at least about 5.0.
 11. The method of claim 7, whereinthe orientation is performed simultaneously in the at least twodirections.
 12. The method of claim 7, wherein the film is firstoriented in one of the at least two directions, and is then orientedsimultaneously in the at least two directions.
 13. The method of claim7, wherein the film is first oriented simultaneously in the at least twodirections, followed by further orientation in at least one of the atleast two directions.
 14. The method of claim 1, wherein saidterephthalic acid polyester has an intrinsic viscosity within the rangeof about 0.6 to about 1.1 dL/g.
 15. The method of claim 1, wherein saidterephthalic acid polyester has an intrinsic viscosity within the rangeof about 0.72 to about 0.95 dL/g.
 16. The method of claim 1, whereinsaid naphthalene dicarboxylic acid polyester has an intrinsic viscositywithin the range of about 0.47 to about 0.63 kg/hr.
 17. A method formaking a multilayer polyester film, comprising the steps of:providing afirst resin comprising polyethylene terephthalate; providing a secondresin comprising polyethylene naphthalate; extruding the first andsecond resin into a multilayer film in which a layer comprising thefirst resin is disposed between layers comprising the second resin; andheating the layer comprising the first resin at a sufficienttemperature, and for a sufficient amount of time, to inducecrystallization in the first resin.
 18. The method of claim 17, whereinthe layer comprising the first resin is stretched after it is heated.19. A method for making a multilayer polyester film, comprising thesteps of:providing a first resin comprising polyethylene terephthalate;providing a second resin comprising polyethylene naphthalate; extrudingthe first and second resin into a multilayer film in which a layercomprising the first resin is disposed between layers comprising thesecond resin; stretching the film in at least the transverse direction;passing the film through at least one heat zone having a temperature ofabout 177° C. to about 235° C.; and relaxing the film in the transversedirection.
 20. The method of claim 19, wherein the film is relaxed inthe transverse direction to a stretch ratio of less than about 4.2. 21.The method of claim 19, wherein the first heat zone has a temperature ofless than about 204° C.
 22. The method of claim 19, wherein the firstheat zone has a temperature of less than about 193° C.
 23. The method ofclaim 19, wherein the film is passed through two heat zones, and whereinthe temperature of the second heat zone is at least about 204° C. 24.The method of claim 19, wherein the film is passed through two heatzones, and wherein the temperature of the second heat zone is at leastabout 216° C.
 25. The method of claim 19, wherein the film is relaxed inthe transverse direction by at least about 0.26 stretch ratio units. 26.The method of claim 19, wherein the film is relaxed in the transversedirection by at least about 0.39 stretch ratio units.
 27. The method ofclaim 19, wherein the film is passed through two heat zones, wherein thetemperature of the first heat zone is less than about 204° C., whereinthe temperature of the second heat zone is at least about 204° C., andwherein the film is relaxed in the transverse direction by at leastabout 0.26 stretch ratio units.
 28. The method of claim 19, wherein thefilm is passed through two heat zones, wherein the temperature of thefirst heat zone is less than about 192° C., wherein the temperature ofthe second heat zone is at least about 216° C., and wherein the film isrelaxed in the transverse direction by at least about 0.39 stretch ratiounits.
 29. A method for improving the modulus of a multilayer film,comprising the steps of:providing a multilayer film comprising first,second and third layers, wherein the second layer comprises polyethyleneterephthalate and is disposed between the first and third layers, andwherein at least one of the first and third layers comprisespolyethylene naphthalate; and stretching the multilayer film to astretch ratio of at least about 5.5.
 30. The method of claim 29, whereinboth the first and third layers comprise polyethylene naphthalate. 31.The method of claim 29, wherein the multilayer film is stretched to astretch ratio of at least about 5.7.
 32. The method of claim 29, whereinthe multilayer film comprises about 70% to about 95% polyethylenenaphthalate.
 33. A method for making a multilayer polyester film,comprising the steps of:providing a first resin comprising polyethyleneterephthalate; providing second and third resins; extruding the first,second, and third resins into a multilayer film in which at least onelayer comprising the first resin is disposed between a layer comprisingthe second resin and a layer comprising the third resin; heating thefilm to crystallize the first resin; and stretching the film in at leastone direction.
 34. The method of claim 33, wherein at least one of thesecond and third resins is an elastomer.
 35. The method of claim 33,wherein at least one of the second and third resins is a naphthalenedicarboxylic acid polyester.
 36. The method of claim 17, wherein themultilayer film contains at least 7 layers which comprise a resinselected from the group consisting of naphthalene dicarboxylic acidpolyesters and terephthalic acid polyesters.
 37. The method of claim 17,wherein the multilayer film contains at least 13 layers which comprise aresin selected from the group consisting of naphthalene dicarboxylicacid polyesters and terephthalic acid polyesters.
 38. The method ofclaim 19, wherein the multilayer film contains at least 7 layers whichcomprise a resin selected from the group consisting of naphthalenedicarboxylic acid polyesters and terephthalic acid polyesters.
 39. Themethod of claim 19, wherein the multilayer film contains at least 13layers which comprise a resin selected from the group consisting ofnaphthalene dicarboxylic acid polyesters and terephthalic acidpolyesters.
 40. The method of claim 33, wherein the multilayer filmcontains at least 7 layers which comprise a resin selected from thegroup consisting of naphthalene dicarboxylic acid polyesters andterephthalic acid polyesters.
 41. The method of claim 33, wherein themultilayer film contains at least 13 layers which comprise a resinselected from the group consisting of naphthalene dicarboxylic acidpolyesters and terephthalic acid polyesters.